Polygon-based bioptical POS scanning system employing dual independent optics platforms disposed beneath horizontal and vertical scanning windows

ABSTRACT

A bioptical laser scanning system employing a plurality of laser scanning stations about a two independently controlled rotating polygonal mirrors. The system has an ultra-compact construction, ideally suited for space-constrained retail scanning environments, and generates a 3-D omnidirectional laser scanning pattern between the bottom and side-scanning windows during system operation. The laser scanning pattern of the present invention comprises a complex of quasi-orthogonal laser scanning planes, including a plurality of substantially-vertical laser scanning planes for reading bar code symbols having bar code elements (i.e. ladder type bar code symbols) that are oriented substantially horizontal with respect to the bottom-scanning window, and a plurality of substantially-horizontal laser scanning planes for reading bar code symbols having bar code elements (i.e. picket-fence type bar code symbols) that are oriented substantially vertical with respect to the bottom-scanning window.

CROSS-REFERENCE TO RELATED U.S. APPLICATIONS

[0001] This is a Continuation-in-Part of copending application Ser. No.09/551,887 entitled “Bioptical Holographic Laser Scanning System” filedApr. 18, 2000, said application being owned by Assignee, MetrologicInstruments, Inc., of Blackwood, N.J., and incorporated herein byreference as if fully set forth herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates generally to laser scanners ofultra-compact design capable of reading bar code symbols inpoint-of-sale (POS) and other demanding scanning environments.

[0004] 2. Brief Description of the Prior Art

[0005] The use of bar code symbols for product and articleidentification is well known in the art. Presently, various types of barcode symbol scanners have been developed. In general, these bar codesymbol readers can be classified into two distinct classes.

[0006] The first class of bar code symbol reader simultaneouslyilluminates all of the bars and spaces of a bar code symbol with lightof a specific wavelength(s) in order to capture an image thereof forrecognition and decoding purposes. Such scanners are commonly known asCCD scanners because they use CCD image detectors to detect images ofthe bar code symbols being read.

[0007] The second class of bar code symbol reader uses a focused lightbeam, typically a focused laser beam, to sequentially scan the bars andspaces of a bar code symbol to be read. This type of bar code symbolscanner is commonly called a “flying spot” scanner as the focused laserbeam appears as “a spot of light that flies” across the bar code symbolbeing read. In general, laser bar code symbol scanners aresub-classified further by the type of mechanism used to focus and scanthe laser beam across bar code symbols.

[0008] The majority of laser scanners in use today, particular in retailenvironments, employ lenses and moving (i.e. rotating or oscillating)mirrors and/or other optical elements in order to focus and scan laserbeams across bar code symbols during code symbol reading operations. Indemanding retail scanning environments, it is common for such systems tohave both bottom and side-scanning windows to enable highly aggressivescanner performance, whereby the cashier need only drag a bar codedproduct past these scanning windows for the bar code thereon to beautomatically read with minimal assistance of the cashier or checkoutpersonal. Such dual scanning window systems are typically referred to as“bioptical” laser scanning systems as such systems employ two sets ofoptics disposed behind the bottom and side-scanning windows thereof.Examples of polygon-based bioptical laser scanning systems are disclosedin U.S. Pat. Nos. 4,229,588 and 4,652,732, assigned to NCR, Inc., eachincorporated herein by reference in its entirety.

[0009] In general, prior art bioptical laser scanning systems aregenerally more aggressive that conventional single scanning windowsystems. For this reason, bioptical scanning systems are often deployedin demanding retail environments, such as supermarkets and high-volumedepartment stores, where high check-out throughput is critical toachieving store profitability and customer satisfaction.

[0010] While prior art bioptical scanning systems represent atechnological advance over most single scanning window system, prior artbioptical scanning systems in general suffered from various shortcomingsand drawbacks.

[0011] In particular, the laser scanning patterns of such prior artbioptical laser scanning systems are not optimized in terms of scanningcoverage and performance, and are generally expensive to manufacture byvirtue of the large number of optical components presently required toconstructed such laser scanning systems.

[0012] Thus, there is a great need in the art for an improvedbioptical-type laser scanning bar code symbol reading system, whileavoiding the shortcomings and drawbacks of prior art laser scanningsystems and methodologies.

[0013] Moreover, the performance of such aggressive laser scanningsystems (in scanning a bar code symbol and accurately produce digitalscan data signals representative of a scanned bar code symbol) issusceptible to noise, including ambient noise, thermal noise and papernoise. More specifically, during operation of such machines, a focusedlight beam is produced from a light source such as a visible laser diode(VLD), and repeatedly scanned across the elements of the code symbolattached, printed or otherwise fixed to the object to be identified. Inthe case of bar code scanning applications, the elements of the codesymbol consists of a series of bar and space elements of varying width.For discrimination purposes, the bars and spaces have different lightreflectivity (e.g. the spaces are highly light-reflective while the barsare highly light-absorptive). As the laser beam is scanned across thebar code elements, the bar elements absorb a substantial portion of thelaser beam power, whereas the space elements reflective a substantialportion thereof. As a result of this scanning process, the intensity ofthe laser beam is modulated to in accordance with the informationstructure encoded within the scanned bar code symbol. As the laser beamis scanned across the bar code symbol, a portion of the reflected lightbeam is collected by optics within the scanner. The collected lightsignal is subsequently focused upon a photodetector within the scannerwhich generates an analog electrical output signal which can bedecomposed into a number of signal components, namely a digital scandata signal having first and second signal levels, corresponding to thebars and spaces within the scanned code symbol; ambient-light noiseproduced as a result of ambient light collected by the light collectionoptics of the system; thermal noise produced as a result of thermalactivity within the signal detecting and processing circuitry; and“paper” or substrate noise produced as a result of the microstructure ofthe substrate in relation to the cross-sectional dimensions of thefocused laser scanning beam. The analog scan data signal haspositive-going transitions and negative-going transitions which signifytransitions between bars and spaces in the scanned bar code symbol.However, as a result of such noise components, the transitions from thefirst signal level to the second signal level and vice versa are notperfectly sharp, or instantaneous. Consequently, it is difficult todetermine the exact instant that each binary signal level transitionoccurs in detected analog scan data signal.

[0014] It is well known that the ability of a scanner to accurately scana bar code symbol and accurately produce digital scan data signalsrepresentative of a scanned bar code symbol in noisy environmentsdepends on the depth of modulation of the laser scanning beam. The depthof modulation of the laser scanning beam, in turn, depends on severalimportant factors, namely: the ratio of the laser beam cross-sectionaldimensions at the scanning plane to the width of the minimal bar codeelement in the bar code symbol being scanned, and (ii) the signal tonoise ratio (SNR) in the scan data signal processing stage where binarylevel (1-bit) analog to digital (A/D) signal conversion occurs.

[0015] As a practical matter, it is not possible in most instances toproduce analog scan data signals with precisely-defined signal leveltransitions. Therefore, the analog scan data signal must be furtherprocessed to precisely determine the point at which the signal leveltransitions occur.

[0016] Hitherto, various circuits have been developed for carrying outsuch scan data signal processing operations. Typically, signalprocessing circuits capable of performing such operations includefilters for removing unwanted noise components, and signal thresholdingdevices for rejecting signal components which do not exceed apredetermined signal level.

[0017] One very popular approach for converting analog scan data signalsinto digital scan data signals is disclosed in U.S. Pat. No. 4,000,397,incorporated herein by reference in its entirety. In this U.S. LettersPatent, a method and apparatus are disclosed for precisely detecting thetime of transitions between the binary levels of encoded analog scandata signals produced from various types of scanning devices. Accordingto this prior art method, the first signal processing step involvesdouble-differentiating the analog scan data input signal S_(analog) toproduce a second derivative signal S″_(analog). Then the zero-crossingsof the second derivative signal are detected, during selected gatingperiods, to signify the precise time at which each transition betweenbinary signal levels occurs. As taught in this U.S. Patent, the selectedgating periods are determined using a first derivative signalS′_(analog) formed by differentiating the input scan data signalS_(analog). Whenever the first derivative signal S′_(analog) exceeds athreshold level using peak-detection, the gating period is present andthe second derivative signal S″_(analog) is detected for zero-crossings.At each time instant when a second-derivative zero-crossing is detected,a binary signal level is produced at the output of the signal processor.The binary output signal level is a logical “1” when the detected signallevel falls below the threshold at the gating interval, and a logical“0” when the detected signal level falls above the threshold at thegating interval. The output digital signal S_(digital) produced by thissignal processing technique corresponds to the digital scan data signalcomponent contributing to the underlying structure of the analog scandata input signal S_(analog).

[0018] While the above-described signal processing technique generates asimple way of generating a digital scan data signal from a correspondinganalog scan data signal, this method has a number of shortcomings anddrawbacks.

[0019] In particular, thermal as well as “paper” or substrate noiseimparted to the analog scan data input signal S_(analog) tends togenerate zero-crossings in the second-derivative signal S″_(analog) inmuch the same manner as does binary signal level transitions encoded inthe input analog scan data signal S_(analog). Consequently, the gatingsignal mechanism disclosed in U.S. Pat. No. 4,000,397 allows “false”second-derivative zero-crossing signals to be passed onto thesecond-derivative zero-crossing detector thereof, thereby producingerroneous binary signal levels at the output stage of this prior artsignal processor. In turn, error-ridden digital data scan data signalsare transmitted to the digital scan data signal processor of the barcode scanner for conversion into digital words representative of thelength of the binary signal levels in the digital scan data signal. Thiscan result in significant errors during bar code symbol decodingoperations, causing objects to be incorrectly identified and/orerroneous data to be entered into a host system.

[0020] Also, when scanning bar code symbols within a large scanningfield with multiple scanning planes that cover varying focal zones ofthe scanning field, as taught in co-applicant's PCT International PatentPublication No. WO 97/22945 published on Jun. 26, 1997, Applicants' haveobserved that the effects of paper/substrate noise are greatly amplifiedwhen scanning bar code symbols in the near focal zone(s), therebycausing a significant decrease in overall system performance. In the farout focal zones of the scanning system, Applicants have observed thatlaser beam spot speed is greatest and the analog scan data signalsproduced therefrom are time-compressed relative to analog scan datasignals produced from bar code symbols scanned in focal zones closer tothe scanning system. Thus, in such prior art laser scanning systems,Applicants' have provided, between the first and second differentiatorstages of the scan data signal processor thereof, a low-pass filter(LHF) having cutoff frequency which passes (to the second differentiatorstage) the spectral components of analog scan data signals produced whenscanning bar code elements at the focal zone furthest out from thescanning system. While this technique has allowed prior art scanningsystems to scan bar codes in the far focal zones of the system, it hasin no way addressed or provided a solution to the problem of increasedpaper/substrate noise encountered when scanning bar code symbols in thenear focal zones of such laser scanning systems.

[0021] Moreover, although filters and signal thresholding devices areuseful for rejecting noise components in the analog scan signal, suchdevices also limit the scan resolution of the system, potentiallyrendering the system incapable of reading low contrast and highresolution bar code symbols on surfaces placed in the scanning field.

[0022] Thus, there is a great need in the art for improved laserscanning system wherein the analog scan data signals generatedtherewithin are processed so that the effects of thermal and paper noiseencountered within the system are significantly mitigated while notcompromising the scan resolution of the system.

OBJECTS AND SUMMARY OF THE PRESENT INVENTION

[0023] Accordingly, a primary object of the present invention is toprovide a novel bioptical laser scanning system which is free of theshortcomings and drawbacks of prior art bioptical laser scanning systemsand methodologies.

[0024] Another object of the present invention is to provide a biopticallaser scanning system, wherein a plurality of pairs of quasi-orthogonallaser scanning planes are projected within predetermined regions ofspace contained within a 3-D scanning volume defined between the bottomand side-scanning windows of the system.

[0025] Another object of the present invention is to provide such abioptical laser scanning system, wherein the plurality of pairs ofquasi-orthogonal laser scanning planes are produced using at least onerotating polygonal mirror having scanning facets that have high and lowelevation angle characteristics.

[0026] Another object of the present invention is to provide such abioptical laser scanning system, wherein the plurality of pairs ofquasi-orthogonal laser scanning planes are produced using at least tworotating polygonal mirrors, wherein a first rotating polygonal mirrorproduces laser scanning planes that project from the bottom-scanningwindow, and wherein a second rotating polygonal mirror produces laserscanning planes that project from the side-scanning window.

[0027] Another object of the present invention is to provide such abioptical laser scanning system, wherein each pair of quasi-orthogonallaser scanning planes comprises a plurality of substantially-verticallaser scanning planes for reading bar code symbols having bar codeelements (i.e., ladder type bar code symbols) that are orientedsubstantially horizontal with respect to the bottom-scanning window, anda plurality of substantially-horizontal laser scanning planes forreading bar code symbols having bar code elements (i.e., picket-fencetype bar code symbols) that are oriented substantially vertical withrespect to the bottom-scanning window.

[0028] Another object of the present invention is to provide a biopticallaser scanning system comprising a plurality of laser scanning stations,each of which produces a plurality of groups of quasi-orthogonal laserscanning planes that are projected within predetermined regions of spacecontained within a 3-D scanning volume defined between the bottom andside-scanning windows of the system.

[0029] Another object of the present invention is to provide a biopticallaser scanning system, wherein two visible laser diodes (VLDs) disposedon opposite sides of a rotating polygonal mirror are used to create aplurality of groups of quasi-orthogonal laser scanning planes thatproject through the bottom-scanning window.

[0030] Another object of the present invention is to provide a biopticallaser scanning system, wherein a single VLD is used to create the scanpattern projected through the side-scanning window.

[0031] Another object of the present invention is to provide a biopticallaser scanning system which generates a plurality of quasi-orthogonallaser scanning planes that project through the bottom-scanning windowand side-scanning window to provide 360 degrees of scan coverage at laPOS station.

[0032] Another object of the present invention is to provide a biopticallaser scanning system which generates a plurality of vertical laserscanning planes that project through the bottom-scanning window toprovide 360 degrees of scan coverage.

[0033] Another object of the present invention is to provide a biopticallaser scanning system which generates a plurality of horizontal andvertical laser scanning planes that project from the top of theside-scanning window downward, which are useful for reading ladder typeand picket-fence type bar code symbols on top-facing surfaces.

[0034] A further object of the present invention is to provide such abioptical laser scanning system, in which an independent signalprocessing channel is provided for each laser diode and lightcollection/detection subsystem in order to improve the signal processingspeed of the system.

[0035] A further object of the present invention is to provide such abioptical laser scanning system, in which a plurality of signalprocessors are used for simultaneously processing the scan data signalsproduced from each of the photodetectors within the laser scanner.

[0036] A further object of the present invention is to provide abioptical laser scanning system that provides improved scan coverageover the volume disposed between the two scanning windows of the system.

[0037] Another object of the present invention is to provide a biopticallaser scanning system that produces horizontal scanning planes capableof reading picket-fence type bar code symbols on back-facing surfaceswhose normals are substantially offset from the normal of theside-scanning window.

[0038] Another object of the present invention is to provide a biopticallaser scanning system that produces horizontal scanning planes thatproject from exterior portions (for example, left side and right side)of the side-scanning window at a characteristic propagation directionwhose non-vertical component is greater than thirty-five degrees fromnormal of the side-scanning window.

[0039] Another object of the present invention is to provide a biopticallaser scanning system that produces vertical scanning planes capable ofreading ladder type bar code symbols on back-facing surfaces whosenormals are substantially offset from the normal of the side-scanningwindow.

[0040] Another object of the present invention is to provide a biopticallaser scanning system that produces a plurality a vertical scanningplanes that project from portions (e.g., back-left and back-rightcorners) of the bottom-scanning window proximate to the back of thebottom-scanning window and the bottom side of the side-scanning window.

[0041] Another object of the present invention is to provide a biopticallaser scanning system that produces vertical scanning planes capable of360 degree reading of ladder type bar code symbols (e.g., on bottom-,front-, left-, back- and/or right-facing surfaces of an article).

[0042] Another object of the present invention is to provide a biopticallaser scanning system that produces a plurality a vertical scanningplanes that project from each one of the four corners of thebottom-scanning window. system Another object of the present inventionis to provide a bioptical laser scanning system that produces at leasteight different vertical scanning planes that project from theside-scanning window.

[0043] Another object of the present invention is to provide a biopticallaser scanning system that produces at least 13 different horizontalscanning planes that project from the side-scanning window.

[0044] Another object of the present invention is to provide a biopticallaser scanning system that produces at least 20 different horizontalscanning planes that project from the

[0045] side-scanning window Another object of the present invention isto provide a bioptical laser scanning system that produces at least 21different scanning planes that project from the side-scanning window.

[0046] Another object of the present invention is to provide a biopticallaser scanning system that produces at least 28 different scanningplanes that project from the side-scanning window.

[0047] Another object of the present invention is to provide a biopticallaser scanning system that produces at least seven different verticalscanning planes that project from the bottom-scanning window.

[0048] Another object of the present invention is to provide a biopticallaser scanning system that produces at least 21 different horizontalscanning planes that project from the bottom-scanning window.

[0049] Another object of the present invention is to provide a biopticallaser scanning system that produces at least 25 different scanningplanes that project from the bottom-scanning window.

[0050] Another object of the present invention is to provide a biopticallaser scanning system with at least one laser beam production modulethat cooperates with a rotating polygonal mirror and a plurality oflaser beam folding mirrors to produce a plurality of scanning planesthat project through the window, wherein the incidence angle of thelaser beam produced by the laser beam production module is offset withrespect to the axis of rotation of the rotating polygonal mirror.

[0051] A further object of the present invention is to provide such abioptical laser scanning system wherein the offset of the incidenceangle of the laser and the axis of rotation of the rotating polygonalmirror produces overlapping scanning ray patterns that are incident onat least one common mirror to provide a dense scanning patternprojecting therefrom.

[0052] In another aspect of the present invention, it is a primaryobjective to provide an improved laser scanning system, wherein scandata signals produced therewithin are processed so that the effects ofthermal and paper noise encountered within the system are significantlymitigated.

[0053] Another object of the present invention is to provide an improvedlaser scanning system having a scan data signal processor with improveddynamic range.

[0054] Another object of the present invention is to provide an improvedlaser scanning system having a multi-path scan data signal processorthat employs different operational characteristics (such as differentfilter cutoff frequencies, peak thresholds, etc) in distinct signalprocessing paths.

[0055] Another object of the present invention is to provide an improvedlaser scanning system having a multi-path scan data signal processorthat concurrently performs distinct signal processing operations thatemploy different operational characteristics (such as different filtercutoff frequencies, peak thresholds, etc).

[0056] Another object of the present invention is to provide an improvedlaser scanning system employing a scan data signal processor having aplurality of processing paths each processing the same data signalderived from the output of a photodetector to detect bar code symbolstherein and generate data representing said bar code symbols, whereinthe plurality of processing paths have different operationalcharacteristics (such as different filter cutoff frequencies, peakthresholds, etc).

[0057] A further object of the present invention is to provide such animproved laser scanning system wherein each signal processing pathincludes a peak detector that identifies time periods during which afirst derivative signal exceeds at least one threshold level, andwherein the at least one threshold level for one of the respective pathsis different than the at least one threshold level for another of therespective paths.

[0058] A further object of the present invention is to provide such animproved laser scanning system wherein each signal processing pathperforms low pass filtering, wherein the cut-off frequency of such lowpass filtering for one of the respective paths is different than thecut-off frequency of such low pass filtering for another of therespective paths.

[0059] A further object of the present invention is to provide such animproved laser scanning system wherein each signal processing pathperforms voltage amplification, wherein the gain of such voltageamplification for one of the respective paths is different than the gainof such voltage amplification for another of the respective paths.

[0060] Another object of the present invention is to provide an improvedlaser scanning system employing a scan data signal processor withdynamic peak threshold levels.

[0061] Another object of the present invention is to provide an improvedlaser scanning system employing a scan data signal processor withmultiple signal processing paths that perform analog signal processingfunctions with analog circuitry.

[0062] Another object of the present invention is to provide an improvedlaser scanning system employing a scan data signal processor withmultiple signal processing paths that perform digital signal processingfunctions with digital signal processing circuitry.

[0063] A further object of the present invention is to provide such alaser scanning system, wherein each processing path is performedsequentially based on real-time status of a working buffer that storesdata values for digital signal processing.

[0064] These and other objects of the present invention will becomeapparent hereinafter and in the claims to Invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] In order to more fully understand the Objects of the PresentInvention, the following Detailed Description of the IllustrativeEmbodiments should be read in conjunction with the accompanying FigureDrawings in which:

[0066]FIG. 1A is a perspective view of an illustrative embodiment of thebioptical laser scanning system of the present invention, showing itsbottom-scanning and side-scanning windows formed with its compactscanner housing.

[0067]FIG. 1B is a side view of the bioptical laser scanning system ofFIG. 1A.

[0068]FIG. 1C is a side view of the bioptical laser scanning system ofFIGS. 1A and 1B with an integrated weigh scale for use in aPoint-Of-Sale (POS) retail environment as shown in FIG. 1H.

[0069]FIG. 1D is a front view of the bioptical laser scanning systemwith integrated weigh scale of FIG. 1C.

[0070]FIG. 1E is a top view of the bioptical laser scanning system withintegrated weigh scale of FIG. 1C.

[0071]FIGS. 1F and 1G are perspective views of a model of the biopticallaser scanning system with integrated weigh scale of FIG. 1C.

[0072]FIG. 1H is a perspective view of the bioptical laser scanningsystem of the present invention shown installed in a Point-Of-Sale (POS)retail environment.

[0073]FIG. 1I is a perspective view of the bioptical laser scanningsystem of the present invention shown installed above a work surface(e.g. a conveyor belt structure) employed, for example, in manualsortation operations or the like.

[0074]FIG. 1J is a pictorial illustration depicting a normal of asurface and the “flip-normal” of the surface as used herein.

[0075]FIG. 1K is a pictorial illustration depicting bottom-facing,top-facing, back-facing, front-facing, left-facing and right-facingsurfaces of a rectangular shaped article oriented within the scanningvolume of the bioptical laser scanning system of the present inventiondisposed between the bottom-scanning and side-scanning windows of thesystem.

[0076]FIG. 2A is a perspective view of a wire frame model of portions ofthe horizontal section of the bioptical laser scanning system of theillustrative embodiment of the present invention, including thebottom-scanning window (e.g., horizontal window), first rotatingpolygonal mirror PM1, and the first and second scanning stations HST1and HST2 disposed thereabout, wherein each laser scanning stationincludes a set of laser beam folding mirrors disposed about the firstrotating polygon PM1.

[0077]FIG. 2B is a top view of the wire frame model of FIG. 2A.

[0078]FIG. 2C1 is a perspective view of a wire frame model of portionsof the horizontal section of the bioptical laser scanning system of theillustrative embodiment of the present invention, including thebottom-scanning window (e.g., horizontal window), first rotatingpolygonal mirror PM1, and the first and second scanning stations HST1and HST2 disposed thereabout, wherein each laser scanning stationincludes a light collecting/focusing optical element (labeled LC_(HST1)and LC_(HST2)) that collects light from a scan region that encompassesthe outgoing scanning planes and focuses such collected light onto aphotodetector (labeled PD_(HST1) and PD_(HST2)), which produces anelectrical signal whose amplitude is proportional to the intensity oflight focused thereon. The electrical signal produced by thephotodetector is supplied to analog/digital signal processing circuitry,associated with the first and second laser scanning station HST1 andHST2, that process analog and digital scan data signals derivedtherefrom to perform bar code symbol reading operations. Preferably, thefirst and second laser scanning stations HST1 and HST2 each include alaser beam production module (not shown) that generates a laser scanningbeam (labeled SB1 and SB2) that is directed through an aperture in thecorresponding light collecting/focusing element as shown to a point ofincidence on the first rotating polygonal mirror PM1.

[0079]FIG. 2C2 is a top view of the wire frame model of FIG. 2C1.

[0080]FIG. 2D is a perspective view of a wire frame model of portions ofthe vertical section of the bioptical laser scanning system of theillustrative embodiment of the present invention, including theside-scanning window (e.g., vertical window), second rotating polygonalmirror PM2, and the third scanning station VST1 disposed thereabout; thethird laser scanning station includes a set of laser beam foldingmirrors disposed about the second rotating polygon PM2.

[0081]FIG. 2E is a top view of the wire frame model of FIG. 2D.

[0082]FIG. 2F is a perspective view of a wire frame model of portions ofthe vertical section of the bioptical laser scanning system of theillustrative embodiment of the present invention, including theside-scanning window (e.g., vertical window), second rotating polygonalmirror PM2, and the third scanning stations VST1 disposed thereabout,wherein the third laser scanning station VST1 includes a lightcollecting/focusing optical element (labeled LC_(VST1)) that collectslight from a scan region that encompasses the outgoing scanning planesand focuses such collected light onto a photodetector (labeledPD_(VST1)), which produces an electrical signal whose amplitude isproportional to the intensity of light focused thereon. The electricalsignal produced by the photodetector is supplied to analog/digitalsignal processing circuitry, associated with the first and second laserscanning station HST1 and HST2, that process analog and digital scandata signals derived therefrom to perform bar code symbol readingoperations. Preferably, the third laser scanning station VST1 includes alaser beam production module (not shown) that generates a laser scanningbeam SB3 that is directed to a small light directing mirror disposed inthe interior of the light collecting/focusing element LC_(VST1) asshown, which redirects the laser scanning beam SB3 to a point ofincidence on the second rotating polygonal mirror PM2.

[0083]FIG. 2G1 depicts the angle of each facet of the rotating polygonalmirrors PM1 and PM2 with respect to the rotational axis of therespective rotating polygonal mirrors in the illustrative embodiment ofthe present invention.

[0084]FIG. 2G2 is a pictorial illustration of the scanning ray patternproduced by the four facets of the first polygonal mirror PM1 inconjunction with the laser beam source provided by the first laserscanning station HST1 in the illustrative embodiment of the presentinvention is shown in FIG. 2G2. A similar scanning ray pattern isproduced by the four facets of the first polygonal mirror PM1 inconjunction with the laser beam source provided by the second laserscanning station HST2.

[0085]FIG. 2G3 is a pictorial illustration of the scanning ray patternproduced by the four facets of the second polygonal mirror PM2 inconjunction with the laser beam source provided by the third laserscanning station VST1 in the illustrative embodiment of the presentinvention; the facets of the second polygonal mirror PM2 can bepartitioned into two classes: a first class of facets (corresponding toangles β₁ and β₂) have High Elevation (HE) angle characteristics, and asecond class of facets (corresponding to angles β₃ and β₄) have LowElevation (LE) angle characteristics; high and low elevation anglecharacteristics are referenced by the plane P1 that contains theincoming laser beam and is normal to the rotational axis of the secondpolygonal mirror PM2; each facet in the first class of facets (havinghigh beam elevation angle characteristics) produces an outgoing laserbeam that is directed above the plane P1 as the facet sweeps across thepoint of incidence of the third laser scanning station VST1; whereaseach facet in the second class of facets (having low beam elevationangle characteristics) produces an outgoing laser beam that is directedbelow the plane P1 as the facet sweeps across the point of incidence ofthe third laser scanning station VST1.

[0086]FIG. 2H depicts the offset between the pre-specified angle ofincidence of the laser beams produced by the laser beam productionmodules of the laser scanning stations HST1 and HST2 and the rotationalaxis of the polygonal mirror PM1 along a direction perpendicular to therotational axis; Such offset provides for spatial overlap in thescanning pattern of light beams produced from the polygonal mirror PM1by these laser beam production modules; such spatial overlap can beexploited such that the overlapping rays are incident on at least onecommon mirror (mh5 in the illustrative embodiment) to provide a densescanning pattern projecting therefrom; in the illustrative embodiment, adense pattern of horizontal planes (groups GH4) is projected from thefront side of the bottom window as is graphically depicted in FIGS. 3F1,3F2 and 4B1 and 4B2.

[0087]FIG. 3A illustrates the intersection of the four groups of laserscanning planes (with 20 total scanning planes in the four groups)produced by the first laser scanning station HST1 on the bottom-scanningwindow 16 in the illustrative embodiment of the present invention.

[0088] FIGS. 3B1 and 3B2 graphically depict a vector-based nomenclaturethat may be used to define horizontal and vertical scanning planes,respectively, that project through the bottom-scanning window 16.

[0089]FIG. 3C1 and 3C2 is a perspective view and top view, respectively,of a wire frame model that illustrates the first group GH1 of laser beamfolding mirrors of the first laser scanning station (HST1), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the right back corner of the bottom-scanningwindow 16 diagonally outward and upward above the front left side (andfront left corner) of the bottom-scanning window 16 as shown.

[0090]FIG. 3D1 and 3D2 is a front view and top view, respectively, of awire frame model that illustrates the second group GH2 of laser beamfolding mirrors of the first laser scanning station (HST1), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different horizontal laser scanningplanes that project from the right side of the bottom-scanning window 16diagonally outward and upward above the left side of the bottom-scanningwindow 16 as shown.

[0091]FIG. 3E1 and 3E2 is a perspective view and top view, respectivelyof a wire frame model that illustrates the third group GH3 of laser beamfolding mirrors of the first laser scanning station (HST1), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the right front corner of the bottom-scanningwindow 16 diagonally outward and upward above the back left side andback left corner of the bottom-scanning window 16 as shown.

[0092]FIG. 3F1 and 3F2 is a front view and side view, respectively, of awire frame model that illustrates the fourth group GH4 of laser beamfolding mirrors of the first laser scanning station (HST1), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate eight different horizontal laser scanningplanes that project from the front side of the bottom-scanning window 16diagonally outward and upward above the back side of the bottom-scanningwindow 16 as shown; note that the first laser scanning station HST1utilizes mirrors MH4 and MH5 (and not MH6) of group GH4 to produce eightdifferent scan planes therefrom.

[0093]FIG. 4A illustrates the intersection of the four groups of laserscanning planes (with 20 total scanning planes in the four groups)produced by the second laser scanning station HST2 on thebottom-scanning window 16 in the illustrative embodiment of the presentinvention.

[0094]FIG. 4B1 and 4B2 is a front view and side view, respectively, of awire frame model that illustrates the first group (GH4) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate eight different horizontal laser scanningplanes that project from the front side of the bottom-scanning window 16diagonally outward and upward above the back side of the bottom-scanningwindow 16 as shown; note that the second laser scanning station HST2utilizes mirrors MH5 and MH6 (and not MH4) of group GH4 to produce eightdifferent scan planes therefrom.

[0095]FIG. 4C1 and 4C2 is a perspective view and top view, respectively,of a wire frame model that illustrates the second group (GH5) of laserbeam folding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the left front corner of the bottom-scanningwindow 16 diagonally outward and upward above the back right side andback right corner of the bottom-scanning window 16 as shown.

[0096]FIG. 4D1 and 4D2 is a front view and top view, respectively, of awire frame model that illustrates the third group (GH6) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different horizontal laser scanningplanes that project from the left side of the bottom-scanning window 16diagonally outward and upward above the right side of thebottom-scanning window 16 as shown.

[0097]FIG. 4E1 and 4E2 is a perspective view and top-view, respectively,of a wire frame model that illustrates the fourth group (GH7) of laserbeam folding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the left back corner of the bottom-scanningwindow 16 diagonally outward and upward above the front right side andfront right corner of the bottom-scanning window 16 as shown.

[0098]FIG. 4F illustrates the vertical scanning planes that project fromthe bottom-scanning window 16; including 4 groups (namely, GH1, GH3, GH5and GH7); groups GH1 and GH5 project from opposing portions (e.g., theback-right and front-left corners of the window 16) of thebottom-scanning window 16, and groups GH3 and GH7 project from opposingportions (e.g., front-right and back-left corners of the window 16) ofthe bottom-scanning window; note that groups GH1 and GH5 aresubstantially co-planar (i.e., quasi co-planar) and groups GH3 and GH7are substantially co-planar (i.e., quasi co-planar), while groups GH1and GH5 are substantially orthogonal (i.e., quasi-orthogonal) to groupsGH3 and GH7, respectively, as shown.

[0099]FIG. 5A illustrates the intersection of the fourteen groups oflaser scanning planes (with 28 total scanning planes in the fourteengroups) produced by the third laser scanning station VST1 on theside-scanning window 18 in the illustrative embodiment of the presentinvention.

[0100] FIGS. 5B1 and 5B2 graphically depict a vector-based nomenclaturethat may be used to define horizontal and vertical scanning planes,respectively, that project through the side-scanning window 18.

[0101]FIG. 5C1 and 5C2 is a front view and top view, respectively, of awire frame model that illustrates the first group (GV1) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low-elevation (LE) scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differentvertical laser scanning planes that project from the left side of theside-scanning window 18 diagonally down and out across thebottom-scanning window 16 above the front right corner of thebottom-scanning window 16 as shown.

[0102]FIG. 5D1 and 5D2 is a perspective view and side view,respectively, of a wire frame model that illustrates the second group(GV2) of laser beam folding mirrors of the third laser scanning station(VST1), which cooperate with the two low-elevation scanning facets ofthe second rotating polygonal mirror PM2 (corresponding to angles β₃ andβ₄ of the second polygonal mirror PM2 in FIG. 2G1) so as to generate twodifferent vertical laser scanning planes that project from the top leftcorner of the side-scanning window 18 downward toward thebottom-scanning window 16 substantially along the left side of thebottom-scanning window 16 as shown.

[0103]FIG. 5E1 and 5E2 is a front view and side view, respectively, of awire frame model that illustrates the third group (GV3) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low-elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top left quadrantof the side-scanning window 18 diagonally down across thebottom-scanning window 16 as shown.

[0104]FIG. 5F1 and 5F2 is a front view and side view, respectively, of awire frame model that illustrates the fourth group (GV4) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top rightquadrant of the side-scanning window 18 diagonally down across thebottom-scanning window 16 as shown.

[0105]FIG. 5G1 and 5G2 is a front view and side view, respectively, of awire frame model that illustrates the fifth group (GV5) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low-elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differentvertical laser scanning planes that project from the top right corner ofthe side-scanning window 18 downward toward the bottom-scanning window16 substantially along the right side of the bottom-scanning window 16as shown.

[0106]FIG. 5H1 and 5H2 is a front view and side view, respectively, of awire frame model that illustrates the sixth group (GV6) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differentvertical laser scanning planes that project from the right side of theside-scanning window 18 diagonally out across the bottom-scanning window16 above the front left corner of the bottom-scanning window 16 asshown.

[0107] FIGS. 5I1 and 5I2 is a front view and side view, respectively, ofa wire frame model that illustrates the seventh group (GV7) of laserbeam folding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that lproject from the top leftquadrant of the side-scanning window 18 outwardly across thebottom-scanning window 16 (substantially parallel to the bottom-scanningwindow 16) as shown.

[0108]FIG. 5J1 and 5J2 is a front view and top view, respectively, of awire frame model that illustrates the eighth group (GV8) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the left side of theside-scanning window 18 outwardly across the bottom-scanning window 16(substantially parallel to the bottom-scanning window 16) as shown; inthe illustrative embodiment, the characteristic direction of propagationof such scanning planes has a non-vertical component whose orientationrelative to the normal of the side-scanning window 18 is greater than 35degrees.

[0109]FIG. 5K1 and 5K2 is a front view and side view, respectively, of awire frame model that illustrates the ninth group (GV9) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly and downward across thebottom-scanning window 16 as shown.

[0110]FIG. 5L1 and 5L2 is a front view and side view, respectively, of awire frame model, that illustrates the tenth group (GV10) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly and sharply downward across thebottom-scanning window 16 as shown.

[0111]FIG. 5M1 and 5M2 is a front view and side view, respectively, of awire frame model that illustrates the eleventh group (GV11) of laserbeam folding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly and sharply downward across thebottom-scanning window 16 as shown.

[0112]FIG. 5N1 and 5N2 is a front view and side view, respectively, of awire frame model that illustrates the twelfth group (GV12) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly across the bottom-scanningwindow 16 (substantially parallel to the bottom-scanning window 16) asshown.

[0113] FIGS. 5O1 and 5O2 is a front view and top view, respectively, ofa wire frame model that illustrates the thirteenth group (GV13) of laserbeam folding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the right side of theside-scanning window 18 outwardly across the bottom-scanning window 16(substantially parallel to the bottom-scanning window 16) as shown; inthe illustrative embodiment, the characteristic direction of propagationof such scanning planes has a non-vertical component whose orientationrelative to the normal of the side-scanning window 118 is greater than35 degrees.

[0114]FIG. 5P1 and 5P2 is a front view and side view, respectively, of awire frame model that illustrates the fourteenth group (GV14) of laserbeam folding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top rightquadrant of the side-scanning window 18 outwardly across thebottom-scanning window 16 (substantially parallel to the bottom-scanningwindow 16) as shown.

[0115]FIG. 6 is an exemplary timing scheme for controlling the biopticallaser scanner of the illustrative embodiment to cyclically generate acomplex omni-directional 3-D laser scanning pattern from both the bottomand side-scanning windows 16 and 18 thereof during the revolutions ofthe scanning polygonal mirrors PM1 and PM2; in this exemplary timingscheme, four sets of scan plane groups (4*[GH1 . . . GH7]) are producedby stations HST1 and HST2 during each revolution of the polygonal mirrorPM1 concurrently with two sets of scan plane groups (2*[GV1 . . . GV14]) produced by station VST1 during a single revolution of the polygonalmirror PM2; this complex omni-directional scanning pattern isgraphically illustrated in FIGS. 3A tough 5P2; the 3-D laser scanningpattern of the illustrative embodiment consists of 68 different laserscanning planes, which cooperate in order to generate a plurality ofquasi-orthogonal laser scanning patterns within the 3-D scanning volumeof the system, thereby enabling true omnidirectional scanning of barcode symbols.

[0116]FIG. 7 is a functional block diagram of an illustrative embodimentof the electrical subsystem of the bioptical laser scanning systemaccording to the present invention, including: Photodetectors (e.g. asilicon photocell) for detection of optical scan data signals generatedby the respective laser scanning stations; analog signal processingcircuitry for processing (e.g., preamplification, bandpass filtering,and A/D conversion) analog scan data signals, digitizing circuitry forconverting the digital scan data signal D₂, associated with each scannedbar code symbol, into a corresponding sequence of digital words (i.e. asequence of digital count values) D₃, and bar code symbol decodingcircuitry that receives the digital word sequences D₃ produced from thedigitizing circuit, and subject it to one or more bar code symboldecoding algorithms in order to determine which bar code symbol isindicated (i.e. represented) by the digital word sequence D₃; aprogrammed microprocessor 61 with a system bus and associated programand data storage memory, for controlling the system operation of thebioptical laser scanner and performing other auxiliary functions and forreceiving bar code symbol character data (provided by the bar codesymbol decoding circuitry); a data transmission subsystem forinterfacing with and transmitting symbol character data and otherinformation to host computer system (e.g. central computer, cashregister, etc.) over a communication link therebetween; and aninput/output interface for providing drive signals to anaudio-transducer and/or LED-based visual indicators used to signalsuccessful symbol reading operations to users and the like, forproviding user input via interaction with a keypad, and for interfacingwith a plurality of accessory devices (such as an external handheldscanner, a display device, a weigh scale, a magnetic card reader and/ora coupon printer as shown).

[0117]FIGS. 8A and 8B are graphical representations of the powerspectrum of an exemplary analog scan data signal produced when laserscanning a bar code symbol within near and far focal zones of a laserscanning system, shown plotted along with the power density spectrum ofthe paper/substrate noise signal produced while laser scanning the barcode symbol on its substrate within such near and far focal zones.

[0118]FIG. 9 is a functional block diagram of an illustrative embodimentof the multi-path scan data signal processor according to the presentinvention, including: signal conditioning circuitry 903 operably coupledbetween a photodetector 902 and a plurality of signal processing paths(two shown as path A and path B) that process the output of the signalconditioning circuitry in parallel; each signal processing pathincludes: a first derivative signal generation circuit 904 having adifferentiator, low pass filter and amplifier therein; a secondderivative signal generation circuit 906 having a differentiatortherein; a first derivative signal threshold-level generation circuit905; and a zero crossing detector 907, data gate 908, and binary-typeA/D signal conversion circuitry 909; each signal processing path hasdifferent operational characteristics (such as different cutofffrequencies in the filtering stages of the first and second derivativesignal generation circuits of the respective paths, different gaincharacteristics in amplifier stages of the first and second derivativesignal generation circuits of the respective paths, and/or differentpositive and negative signal thresholds in the first derivativethreshold circuitry of the respective paths); the varying operationalcharacteristics of the paths provide different signal processingfunctions.

[0119]FIGS. 10A through 10I are signal diagrams that illustrate theoperation of the multi-path scan data signal processor 901 of theillustrative embodiment of FIG. 9. FIGS. 10A and 10B depict the signalproduced at the output of the photodetector 902 as the laser scanningbeam scans across a bar code symbol; FIG. 10C depicts the output signalproduced by the signal conditioning circuitry 903; and FIGS. 10D through10I depict the processing performed in one of the respective paths ofthe multi-path scan data signal processor 901; similar processingoperations with different operations characteristics are performed inother paths of the multi-path scan data signal processor 901.

[0120]FIG. 11 is a schematic diagram illustrating an exemplaryembodiment of the signal conditioning circuitry 903 of FIG. 9, whichoperates to amplify and smooth out or otherwise filter the scan datasignal produced by the photodetector 902 to remove unwanted noisecomponents therein, including a number of subcomponents arranged in aserial manner, namely: a high gain amplifier stage 1103, a multistageamplifier stage 1105, a differential amplifier stage 1107 and a low passfilter (LPF) stage 1109.

[0121]FIG. 12 is a schematic diagram illustrating an exemplaryimplementation of the first derivative signal generation circuitry 904,which is suitable for use in the two different paths of the scan datasignal processor of FIG. 9, including a number of subcomponents arrangedin a serial manner that process the analog scan data signal produced bythe signal conditioning circuitry 903, namely: a differentiator stage1201, a low-pass filter (LPF) stage 1203, and an amplifier stage 1205.

[0122]FIG. 13 is a schematic diagram illustrating an exemplaryimplementation of the second derivative signal generation circuitry 906,which is suitable for use in the two different paths of the scan datasignal processor of FIG. 9, including: a differentiator stage 1301 thatgenerates a signal whose voltage level is proportional to the derivativeof the first derivative signal produced by the first derivativegeneration circuitry 904 (thus proportional to the second derivative ofthe analog scan data signal produced by the signal conditioningcircuitry 903) for frequencies in a predetermined frequency band.

[0123]FIG. 14 is a schematic diagram illustrating an exemplaryimplementation of the first derivative signal threshold circuitry 905,which is suitable for use in the two different paths of the scan datasignal processor of FIG. 9, including: an amplifier stage 1401 thatamplifies the voltage levels of the first derivative signal produced bythe first derivative signal generation circuitry 904, positive andnegative peak detectors 1403 and 1405, and a comparator stage 1407 thatgenerates output signals (e.g., the Upper_Threshold Signal andLower_Threshold Signal) that indicate the time period when the positiveand negative peaks of the amplified first derivative signal produced bythe amplifier stage exceed predetermined thresholds (i.e., a positivepeak level PPL and a negative peak level NPL).

[0124]FIG. 15 illustrates an exemplary implementation of a zero crossingdetector 907, which is suitable for use in the two different paths ofthe scan data signal processor of FIG. 9, including a comparator circuitthat compares the second derivative signal produced from the secondderivative generation circuit in its respective path with a zero voltagereference (i.e. the AC ground level) provided by the zero referencesignal generator, in order to detect the occurrence of eachzero-crossing in the second derivative signal, and provide outputsignals (ZC_1 and ZC_2 signals) identifying zero crossings in the secondderivative signal.

[0125]FIG. 16 is a schematic diagram illustrating an exemplaryimplementation of the data gating circuitry 908 and 1-Bit A/D conversioncircuitry 909, which is suitable for use in the two different paths ofthe scan data signal processor of FIG. 9.

[0126]FIG. 17A is a functional block diagram of a system architecturesuitable for a digital implementation of the scan data signal processorof the present invention, including: signal conditioning circuitry 1703(which amplifies and filters the analog signal to remove unwanted noisecomponents as described above), analog-to-digital conversion circuitry1705 which samples the conditioned analog scan data signals at asampling frequency at least two times the highest frequency componentexpected in the analog scan data signal, in accordance with the wellknown Nyquist criteria, and quantizes each time-sampled scan data signalvalue into a discrete signal level using a suitable length numberrepresentation (e.g. 8 bits) to produce a discrete scan data signal; andprogrammed processor (e.g., a digital signal processor 1707 andassociated memory 1709 as shown) that processes the discrete signallevels to generate a sequence of digital words (i.e. a sequence ofdigital count values) D₃, each representing the time length associatedwith the signal level transitions in the corresponding digital scan datasignal as described above.

[0127]FIGS. 17B through 17D are functional block diagrams thatillustrate exemplary digital implementations of the multi-path scan dataprocessing according to the present invention, wherein digital signalprocessing operations are preferably carried out on the discrete scandata signal levels generated by the A/D converter 1705 and stored in thememory 1709 of FIG. 17A; FIG. 17B illustrates exemplary digital signalprocessing operations that identify a data frame (e.g., a portion of thediscrete scan data signal levels stored in memory 1709) that potentiallyrepresents a bar code symbol (block 1723) and stores the data frame in aworking buffer (block 1725); FIG. 17C illustrates exemplary digitalsignal processing operations that carry out multi-path scan data signalprocessing according to the present invention; and FIG. 17D illustratesalternative digital signal processing operations that carry outmulti-subpath scan data signal processing (with different firstderivative threshold processing performed in each subpath) according tothe present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS OF THE PRESENTINVENTION

[0128] Referring to the figures in the accompanying Drawings, thevarious illustrative embodiments of the bioptical laser scanner of thepresent invention will be described in great detail.

[0129] In the illustrative embodiments, the apparatus of the presentinvention is realized in the form of an automatic code symbol readingsystem having a high-speed bioptical laser scanning mechanism as well asa scan data processor for decode processing scan data signals producedthereby. However, for the sake of convenience of expression, the term“bioptical laser scanner” shall be used hereinafter to denote the barcode symbol reading system which employs the bioptical laser scanningmechanism of the present invention.

[0130] As shown in FIGS. 1A through 1G, the bioptical laser scanner 1 ofthe illustrative embodiment of the present invention has a compacthousing 2 having a first housing portion 4A and a second housing portion4B which projects from one end of the first housing portion 4A in asubstantially orthogonal manner. When the laser scanner 1 is installedwithin a counter-top surface, as shown in FIG. 1H, the first housingportion 4A oriented horizontally, whereas the second housing portion 4Bis oriented vertically with respect to the POS station. Thus throughoutthe Specification and claims hereof, the terms first housing portion andhorizontally-disposed housing portion may be used interchangeably butrefer to the same structure; likewise, the terms the terms secondhousing portion and vertically-disposed housing portion may be usedinterchangeably but refer to the same structure.

[0131] In the illustrative embodiment, the first housing portion 4A(which includes the bottom-scanning window 16) has width, length andheight dimensions of 11.405, 14.678 and 3.93 inches, respectively,whereas the second housing portion 4B (which includes the side-scanningwindow 18) has width and height dimensions of 12.558 inches and 7.115inches, respectively. The total height of the scanner housing 2 is11.044 inches. In addition, the bottom-scanning window 16 has width andlength dimensions of approximately 3.94 inches (100 mm) and 5.9 inches(150 mm), respectively, to provide a window with a square area ofapproximately 15,000 square mm. And, the side-scanning window 18 haswidth and height dimensions of approximately 9.8 inches (248 mm) and 5.9inches (150 mm), respectively, to provide a window with a square area ofapproximately 37,200 square mm. As will be described in greater detailbelow, the bioptical laser scanning mechanism housed within thisultra-compact housing produces an omni-directional laser scanningpattern within the three-dimensional volume above the bottom-scanningwindow 16 and in front of the side-scanning window 18.

[0132] The omni-directional scanning pattern is capable of readingpicket-fence type bar code symbols on bottom-facing surfaces (i.e., asurface whose normal is directed toward the bottom-scanning window 16 ofthe scanner), top-facing surfaces (i.e., a surface whose “flip-normal”is directed toward the bottom-scanning window 16 of the scanner),back-facing surfaces (i.e., a surface whose normal is directed towardthe side-scanning window 18 of the scanner), front-facing surfaces(i.e., a surface whose “flip-normal” is directed toward theside-scanning window 18 of the scanner), left-facing surfaces (i.e., asurface whose normal is directed toward or above the left side of thescanner), and right-facing surfaces (i.e., a surface whose normal isdirected toward or above the right side of the scanner). A “flip-normal”as used above is a direction co-linear to the normal of a surface yetopposite in direction to this normal as shown in FIG. 1J. An example ofsuch bottom-facing, top-facing, back-facing, front-facing surfaces,left-facing surfaces, and right-facing surfaces of a rectangular shapedarticle oriented in the scan volume of the bioptical laser scanningsystem disposed between bottom-scanning and side-scanning windows of thesystem is illustrated in FIG. 1K.

[0133] The bioptical laser scanning system of the present invention canbe used in a diverse variety of bar code symbol scanning applications.For example, the bioptical laser scanner 1 can be installed within thecountertop of a point-of-sale (POS) station as shown in FIG. 2H. In thisapplication, it is advantageous to integrate a weight scale with thelaser scanning mechanism. Such a device is shown in FIGS. 1C and 1Dincluding a weight transducer 35 affixed to a flange 137 mounted to thefront side of the first housing portion 4A. A weigh platter 31 (uponwhich goods or articles to be weighed are placed) is mechanicallysupported via posts (preferably disposed on its periphery) to a plasticinsert 33. The posts (not shown) and plastic insert 33 transfer theweight forces exerted by the goods or articles placed on the weighplatter 31 to the weight transducer 35 for measurement. In addition, theplastic insert 33 supports the bottom-scanning window 16.

[0134] As shown in FIG. 1H, the bioptical laser scanner 1 can beinstalled within the countertop of a point-of-sale (POS) station 26,having a computer-based cash register 20, a weigh-scale 22 mountedwithin the counter adjacent the laser scanner, and an automatedtransaction terminal (ATM) supported upon a courtesy stand in aconventional manner.

[0135] Alternatively, as shown in FIG. 11, the bioptical laser scannercan be installed above a conveyor belt structure as part of amanually-assisted parcel sorting operation being carried out, forexample, during inventory control and management operations.

[0136] As shown in FIGS. 2A through 2F, the bioptical scanning system 1of the illustrative embodiment includes two sections: a first section(sometimes referred to as the horizontal section) disposed within thefirst housing portion 4A and a second section (sometimes referred to asthe vertical section) substantially disposed within the second housingportion 4B. It should be noted that in the illustrative embodiment,parts of the vertical section are disposed within the back of the firsthousing portion 4A as will become evident from the figures andaccompanying description that follows.

[0137] As shown in FIGS. 2A through 2C2 (and in tables I and II below),the first section includes a first rotating polygonal mirror, and firstand second scanning stations (indicated by HST1 and HST2, respectively)disposed thereabout. The first and second laser scanning stations HST1and HST2 each include a laser beam production module (not shown), a setof laser beam folding mirrors, a light collecting/focusing mirror; and aphotodetector. The first and second laser scanning stations HST1 andHST2 are disposed opposite one another about the first rotatingpolygonal mirror PM1. Each laser scanning station generates a laserscanning beam (shown as SB1 and SB2 in FIG. 2C1 and 2C2) that isdirected to a different point of incidence on the first rotatingpolygonal mirror PM1. The incident laser beams (produced by the firstand second laser scanning stations HST1 and HST2) are reflected by eachfacet (of the first polygonal mirror PM1) at varying angles as the firstpolygonal mirror PM1 rotates to produce two scanning beams (SB1 and SB2)whose direction varies over the rotation cycle of the first polygonalmirror PM1. The first and second laser scanning stations HST1 and HST2include groups of laser beam folder mirrors arranged about the firstpolygonal mirror PM1 so as to redirect the two scanning beams SB1 andSB2 to thereby generate and project different groups of laser scanningplanes through the bottom-scanning window 16. TABLE I Mirror Positions -Horizontal Section (mm): Vertex X Y Z mh1 1 115.25 18.87 3.06 2 109.099.19 42.85 3 99.81 69.42 40.73 4 105.97 79.10 0.94 5 6 7 8 mh2 1 123.91−78.90 2.61 2 95.43 −62.89 39.73 3 95.43 3.57 39.73 4 123.91 19.57 2.615 6 7 8 mh3 1 103.74 −140.29 25.40 2 96.02 −133.84 47.43 3 99.04 −68.0937.13 4 114.48 −80.98 −6.92 5 112.97 −113.85 −1.78 6 7 8 mh4 1 62.08−136.87 −11.25 2 66.99 −152.92 31.34 3 26.71 −165.23 31.34 4 21.80−149.19 −11.25 5 6 7 8 mh5 1 −20.00 −135.31 −11.19 2 −20.00 −148.2427.91 3 20.00 −148.24 27.91 4 20.00 −135.31 −11.19 5 6 7 8 mh6 1 −62.08−136.87 −11.25 2 −66.99 −152.92 31.34 3 −26.71 −165.23 31.34 4 −21.80−149.19 −11.25 5 6 7 8 mh7 1 −96.02 −133.84 47.43 2 −99.04 −68.09 37.133 −114.48 −80.98 −6.92 4 −112.97 −113.85 −1.78 5 −103.74 −140.29 25.40 67 8 mh8 1 −123.91 −78.90 2.61 2 −95.43 −62.89 39.73 3 −95.43 3.57 39.734 −123.91 19.57 2.61 5 6 7 8 mh9 1 −115.25 18.87 3.06 2 −109.09 9.1942.85 3 −99.81 69.42 40.73 4 −105.97 79.10 0.94 5 6 7 8 mh10 1 53.6923.10 −11.94 2 14.23 28.69 8.47 3 47.54 67.87 24.47 4 72.59 81.43 24.475 102.20 77.24 9.16 6 106.06 65.68 −1.17 7 83.67 39.33 −11.94 8 mh11 1123.91 −79.28 2.61 2 75.02 −71.42 −10.49 3 75.02 11.97 −10.49 4 123.9119.83 2.61 5 6 7 8 mh12 1 116.06 −105.01 −10.87 2 43.62 −99.13 −10.90 365.09 −142.38 30.61 4 101.96 −145.37 30.63 5 6 7 8 mh13 1 −101.96−145.37 30.63 2 −65.09 −142.38 30.61 3 −43.62 −99.13 −10.90 4 −116.06−105.01 −10.87 5 6 7 8 mh14 1 −75.02 11.97 −10.49 2 −75.02 −71.42 −10.493 −123.91 −79.28 2.61 4 −123.91 19.83 2.61 5 6 7 8 mh15 1 −54.15 22.24−10.80 2 −84.14 38.47 −10.80 3 −106.53 64.81 −0.04 4 −102.66 76.38 10.305 −73.05 80.57 25.61 6 −48.00 67.01 25.61 7 −14.70 27.83 9.60 8

[0138] TABLE II Scan Line Groups - Horizontal Section Group Mirrors inScanning Station/Scan Identifier Group Lines Type gh1 mh1, mh10 HST1/4vertical gh2 mh2, mh11 HST1/4 horizontal gh3 mh3, mh12 HST1/4 verticalgh4 mh4 HST1/4 horizontal mh5 HST1, HST2/8 mh6 HST2/4 gh5 mh7, mh13HST2/4 vertical gh6 mh8, HST2/4 horizontal mh14 gh7 mh9, mh15 HST2/4vertical

[0139] In addition, as shown in FIGS. 2C1 and 2C2, the first and secondlaser scanning stations HST1 and HST2 each include a lightcollecting/focusing optical element, e.g. parabolic light collectingmirror or parabolic surface emulating volume reflection hologram(labeled LC_(HST1) and LC_(HST2)) that collects light from a scan regionthat encompasses the outgoing scanning planes (produced by the first andsecond laser scanning stations HST1 and HST2) and focuses such collectedlight onto a photodetector (labeled PD_(HST1) and PD_(HST2)), whichproduces an electrical signal whose amplitude is proportional to theintensity of light focused thereon. The electrical signal produced bythe photodetector is supplied to analog/digital signal processingcircuitry, associated with the first and second laser scanning stationHST1 and HST2, that process analog and digital scan data signals derivedtherefrom to perform bar code symbol reading operations. Preferably, thefirst and second laser scanning stations HST1 and HST2 each include alaser beam production module (not shown) that generates a laser scanningbeam (labeled SB1 and SB2) that is directed (preferably through anaperture in the corresponding light collecting/focusing element as shownin FIGS. 2C1 and 2C2) to a point of incidence on the first rotatingpolygonal mirror PM1.

[0140] As shown in FIGS. 2D through 2F and in tables III and IV below,the second section includes a second rotating polygonal mirror PM2 and athird scanning station (denoted VST1) that includes a laser beamproduction module (not shown), a set of laser beam folding mirrors, alight collecting/focusing mirror, and a photodetector. The third laserscanning station VST1 generates a laser scanning beam (labeled as SB3 inFIG. 2F) that is directed to a point of incidence on the second rotatingpolygonal mirror PM2. The incident laser beam is reflected by each facet(of the second polygonal mirror PM2) at varying angles as the secondpolygonal mirror PM2 rotates to produce a scanning beam whose directionvaries over the rotation cycle of the second polygonal mirror PM2. Thethird laser scanning station VST1 includes a set of laser beam foldermirrors arranged about the second rotating polygonal mirror PM2 so as toredirect the scanning beam to thereby generate and project differentgroups of laser scanning planes through the side-scanning window 18.TABLE III Mirror Positions - Vertical Section (mm): Vertex X Y Z mv1 1−74.79 88.94 −10.38 2 −114.09 88.94 16.17 3 −114.09 154.82 16.17 4−74.79 154.82 −10.38 5 6 7 8 mv2 1 −61.12 131.03 −6.76 2 −77.92 146.4225.78 3 −43.75 183.72 25.78 4 −33.41 174.24 5.74 5 −31.44 163.43 −6.76 67 8 mv3 1 −29.78 160.24 −1.35 2 −34.38 185.43 27.65 3 −0.04 184.24 27.654 −0.04 159.21 −1.35 5 6 7 8 mv4 1 0.04 159.21 −1.35 2 0.04 184.24 27.653 34.38 185.43 27.65 4 29.78 160.24 −1.35 5 6 7 8 mv5 1 61.12 131.03−6.76 2 31.44 163.43 −6.76 3 33.41 174.24 5.74 4 43.75 183.72 25.78 577.92 146.42 25.78 6 7 8 mv6 1 74.79 88.94 −10.38 2 74.79 154.82 −10.383 114.09 154.82 16.17 4 114.09 88.94 16.17 5 6 7 8 mv7 1 −107.52 89.3530.99 2 −110.94 68.34 59.03 3 −136.32 120.65 95.14 4 −132.90 141.6667.10 5 6 7 8 mv8 1 −129.50 196.36 99.91 2 −139.66 144.56 68.88 3−133.18 126.69 96.58 4 −123.02 178.48 127.62 5 6 7 8 mv9 1 −42.26 185.7373.40 2 −65.99 163.92 49.03 3 −69.45 141.18 82.25 4 −45.72 162.99 106.625 6 7 8 mv10 1 0.00 190.18 78.00 2 −40.33 183.35 74.96 3 −46.98 168.27105.79 4 0.00 176.23 109.33 5 6 7 8 mv11 1 0.00 176.23 109.33 2 46.98168.27 105.79 3 40.33 183.35 74.96 4 0.00 190.18 78.00 5 6 7 8 mv12 142.26 185.73 73.40 2 45.72 162.99 106.62 3 69.45 141.18 82.25 4 65.99163.92 49.03 5 6 7 8 mv13 1 139.66 144.56 68.88 2 129.50 196.36 99.91 3123.02 178.48 127.62 4 133.18 126.69 96.58 5 6 7 8 mv14 1 132.90 141.6667.10 2 136.32 120.65 95.14 3 110.94 68.34 59.03 4 107.52 89.35 30.99 56 7 8 mv15 1 −59.72 111.27 102.01 2 −38.96 95.77 87.32 3 −42.25 116.9860.28 4 −79.46 144.76 86.61 5 −77.49 132.11 102.74 6 7 8 mv16 1 37.7388.59 93.83 2 29.22 119.90 64.12 3 −29.22 119.90 64.12 4 −37.73 88.5993.83 5 6 7 8 mv17 1 42.25 116.98 60.28 2 38.96 95.77 87.32 3 59.72111.27 102.01 4 79.46 144.76 86.61 5 42.25 116.98 60.28 6 7 8 mv18 1−63.87 149.13 93.46 2 −79.68 162.64 67.06 3 −100.06 208.14 102.55 4−84.26 194.63 128.95 5 6 7 8 mv19 1 −140.43 92.77 119.03 2 −140.43126.87 119.12 3 −136.72 174.44 128.44 4 −125.11 154.96 157.07 5 −130.4187.14 143.79 6 7 8 mv20 1 63.87 149.13 93.46 2 79.68 162.64 67.06 3100.06 208.14 102.55 4 84.26 194.63 128.95 5 6 7 8 mv21 1 130.41 87.14143.79 2 125.11 154.96 157.07 3 136.72 174.44 128.44 4 140.43 126.87119.12 5 140.43 92.77 119.03 6 7 8 mv22 1 −134.07 126.69 200.27 2−103.99 134.04 208.61 3 −94.62 209.63 108.20 4 −124.70 202.28 99.86 5 67 8 mv23 1 94.62 209.63 108.20 2 103.99 134.04 208.61 3 134.07 126.69200.27 4 124.70 202.28 99.86 5 6 7 8 mv24 1 −61.13 193.21 119.96 2−97.12 187.87 131.32 3 −97.12 169.38 170.59 4 −19.20 152.51 206.45 519.20 152.51 206.45 6 97.12 169.38 170.59 7 97.12 187.87 131.32 8 61.13193.21 119.96 mv25 1 −106.74 171.66 177.19 2 −83.23 85.77 217.46 3 0.0085.77 246.33 4 0.00 150.54 222.12 5 6 7 8 mv26 1 0.00 150.54 222.12 20.00 150.54 222.12 3 83.23 85.77 217.46 4 106.74 171.66 177.19 5 6 7 8

[0141] TABLE IV Scan Line Groups - Vertical Section Scanning GroupStation/Scan Identifier Mirrors in Group Lines Type gv1 mv1, mv22 VST1/4vertical left gv2 mv2, mv26 VST1/4 top-down vertical gv3 mv3, mv25VST1/4 top-down horizontal gv4 mv4, mv26 VST1/4 top-down horizontal gv5mv5, mv25 VST1/4 top-down vertical gv6 mv6, mv23 VST1/4 vertical rightgv7 mv7, mv24 VST1/4 high horizontal left gv8 mv8, VST1/4 sidehorizontal left mv18, mv19 gv9 mv9, mv17, VST1/4 low horizontal leftmv24 gv10 mv10, mv16, VST1/4 top-down horizontal mv26 gv11 mv11, mv16,VST1/4 top-down horizontal mv25 gv12 mv12, mv15, VST1/4 low horizontalright mv24 gv13 mv13, mv20, VST1/4 side horizontal right mv21 gv14 mv14,mv24 VST1/4 high horizontal right

[0142] In addition, as shown in FIG. 2F, the third laser scanningstation VST1 includes a light collecting/focusing optical element, e.g.parabolic light collecting mirror or parabolic surface emulating volumereflection hologram (labeled LC_(VST1)) that collects light from a scanregion that encompasses the outgoing scanning planes (produced by thethird laser scanning station VST1) and focuses such collected light ontoa photodetector (labeled PD_(VST1)), which produces an electrical signalwhose amplitude is proportional to the intensity of light focusedthereon. The electrical signal produced by the photodetector is suppliedto analog/digital signal processing circuitry, associated with the thirdlaser scanning station VST1, that process analog and digital scan datasignals derived therefrom to perform bar code symbol reading operations.Preferably, the third laser scanning station VST1 includes a laser beamproduction module (not shown) that generates a laser scanning beam SB3that is directed to a small light directing mirror disposed in theinterior of the light collecting/focusing element LC_(VST1), whichredirects the laser scanning beam SB3 to a point of incidence on thesecond rotating polygonal mirror PM2.

[0143] In the illustrative embodiment, the first polygonal mirror PM1includes 4 facets that are fused in conjunction with the two independentlaser beam sources provided by the first and second laser scanningstations HST1 and HST2 so as project from the bottom-scanning window anomni-directional laser scanning pattern consisting of 40 laser scanningplanes that are cyclically generated as the first polygonal mirror PM1rotates. Moreover, the second polygonal mirror PM2 includes 4 facetsthat are used in conjunction with the independent laser beam sourceprovided by the third laser scanning station VST1 so as to project fromthe side-scanning window an omni-directional laser scanning patternconsisting of 28 laser scanning planes cyclically generated as thesecond polygonal mirror PM2 rotates. Thus, the bioptical laser scanningsystem of the illustrative embodiment project from the bottom andside-scanning windows an omni-directional laser scanning patternconsisting of 68 laser scanning planes cyclically generated as the firstand second polygonal mirrors PM1 and PM2 rotate. It is understood,however, these number may vary from embodiment to embodiment of thepresent invention and thus shall not form a limitation thereof.

[0144]FIG. 2G1 depicts the angle of each facet of the rotating polygonalmirrors PM1 and PM2 with respect to the rotational axis of therespective rotating polygonal mirrors in this illustrative embodiment.The scanning ray pattern produced by the four facets of the firstpolygonal mirror PM1 in conjunction with the laser beam source providedby the first laser scanning station HST1 in the illustrative embodimentis shown in FIG. 2G2. A similar scanning ray pattern is produced by thefour facets of the first polygonal mirror PM1 in conjunction with thelaser beam source provided by the second laser scanning station HST2. Inthe illustrative embodiment of the present invention, the secondrotating polygonal mirror PM2 has two different types of facets based onbeam elevation angle characteristics of the facet. The scanning raypattern produced by the four facets of the second polygonal mirror PM2in conjunction with the laser beam source provided by the third laserscanning station VST1 in the illustrative embodiment is shown in FIG.2G3. The facets of the second polygonal mirror PM2 can be partitionedinto two classes: a first class of facets (corresponding to angles β₁and β₂) have High Elevation (HE) angle characteristics, and a secondclass of facets (corresponding to angles β₃ and β₄) have Low Elevation(LE) angle characteristics. As shown in FIGS. 2G2, high and lowelevation angle characteristics are referenced by the plane P1 thatcontains the incoming laser beam and is normal to the rotational axis ofthe second polygonal mirror PM2. Each facet in the first class of facets(having high beam elevation angle characteristics) produces an outgoinglaser beam that is directed above the plane P1 as the facet sweepsacross the point of incidence of the third laser scanning station VST1.Whereas each facet in the second class of facets (having low beamelevation angle characteristics) produces an outgoing laser beam that isdirected below the plane P1 as the facet sweeps across the point ofincidence of the third laser scanning station VST1. As will becomeapparent hereinafter, the use of scanning facets having such diverseelevation angle characteristics enables an efficient design andconstruction of the second section of the bioptical laser scanning—theplurality of beam folding mirrors used therein can be compactly arrangedwithin a minimized region of volumetric space. Such efficient spacesaving designs are advantageous in space-constricted POS-type scanningapplications.

[0145] In the illustrative embodiment of the present invention, thefirst laser scanning station (HST1) includes four groups of laser beamfolding mirrors (GH1, GH2, GH3, and GH4 as depicted in Table II above)which are arranged about the first rotating polygonal mirror PM1, landcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate and project four different groups of laserscanning planes (with 20 total scanning planes in the four groups)through the bottom-scanning window 16, as graphically illustrated inFIGS. 3A-3G. Note that the first laser scanning station HST1 utilizesmirrors MH4 and MH5 (and not MH6) of group GH4 to produce 8 differentscan planes therefrom. The second laser scanning station (HST2) includesfour groups of laser beam folding mirrors (GH4, GH5, GH6 and GH7 asdepicted in Table II) which are arranged about the first rotatingpolygonal mirror PM1, and cooperate with the four scanning facets of thefirst rotating polygonal mirror so as to generate and project fourdifferent groups of laser scanning planes (with 20 total scanning planesin the four groups) through the bottom-scanning window 16, asgraphically illustrated in FIGS. 4A-4F. Note that the second laserscanning station HST2 utilizes mirrors MH5 and MH6 (and not MH4) ofgroup GH4 to produce 8 different scan planes therefrom. Finally, thethird laser scanning station (VST1) includes fourteen groups of laserbeam folding mirrors (GV1, GV2 . . . GV14 as depicted in Table IV above)which are arranged about the second rotating polygonal mirror PM2, andcooperate with the four scanning facets of the second rotating polygonalmirror PM2 so as to generate and project fourteen different groups oflaser scanning planes (with 28 total scanning planes in the fourteengroups) through the side-scanning window 18, as graphically illustratedin FIGS. 5A-5Q.

[0146] For purposes of illustration and conciseness of description, eachlaser beam folding mirror in each mirror group as depicted in the secondcolumn of Tables II and IV, respectively, is referred to in thesequential order that the outgoing laser beam reflects off the mirrorsduring the laser scanning plane generation process (e.g., the firstmirror in the column causes an outgoing laser beam to undergo its firstreflection after exiting a facet of the rotating polygonal mirror, thesecond mirror in the column causes the outgoing laser beam to undergoits second reflection, etc.).

[0147] First Laser Scanning Station HST1

[0148] As shown in FIGS. 2A, 2B and 3A-3F2, the first laser scanningstation (HST1) includes four groups of laser beam folding mirrors (GH1,GH2, GH3 and GH4) which are arranged about the first rotating polygonalmirror PM1, and cooperate with the four scanning facets of the firstrotating polygonal mirror PM1 so as to generate and project fourdifferent groups of laser scanning planes (with 20 total scanning planesin the four groups) through the bottom-scanning window 16. Theintersection of the four groups of laser scanning planes (with 20 totalscanning planes in the four groups) on the bottom-scanning window 16 isshown in FIG. 3A. The twenty laser scanning planes (of these four groupsprojected through the bottom-scanning window 16) can be classified aseither vertical scanning planes or horizontal scanning planes, which canbe defined as follows.

[0149] As shown in FIGS. 3B1 and 3B2, a scanning plane has acharacteristic direction of propagation D_(p) and a normal directionSP_(N), which define a direction O that is orthogonal thereto (e.g.,O=D_(p)×SP_(N)). For the sake of description, the characteristicdirection of propagation D_(p) of a scanning plane can be defined as themean propagation direction for a plurality of rays that make up thescanning plane. A horizontal scanning plane is a scanning plane whereinthe angle φ between the direction O and the plane defined by thebottom-scanning window 16 is in the range between 0 and 45 degrees (andpreferably in the range between 0 and 20 degrees, and more preferably inthe range between 0 and 10 degrees). An exemplary horizontal scanningplane is shown in FIG. 3B1. A vertical scanning plane is a scanningplane wherein the angle φ between the direction O and the plane definedby the bottom-scanning window 16 is in the range between 45 and 90degrees (and preferably in the range between 70 and 90 degrees, and morepreferably tin the range between 80 and 90 degrees). An exemplaryvertical scanning plane is shown in FIG. 3B2.

[0150] FIGS. 3C1 and 3C2 illustrate the first group GH1 of laser beamfolding mirrors of the first laser scanning station (HST1), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the right back corner of the bottom-scanningwindow 16 diagonally outward and upward above the front left side (andfront left corner) of the bottom-scanning window 16 as shown. Thesescanning planes are useful for reading ladder type bar code symbolsdisposed on bottom-, back-, and right-facing surfaces.

[0151] FIGS. 3D1 and 3D2 illustrate the second group GH2 of laser beamfolding mirrors of the first laser scanning station (HST1), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different horizontal laser scanningplanes that project from the right side of the bottom-scanning window 16diagonally outward and upward above the left side of the bottom-scanningwindow 16 as shown. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on bottom- and right-facingsurfaces.

[0152] FIGS. 3EI and 3E2 illustrate the third group GH3 of laser beamfolding mirrors of the first laser scanning station (HST1), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the right front corner of the bottom-scanningwindow 16 diagonally outward and upward above the back left side andback left corner of the bottom-scanning window 16 as shown. Thesescanning planes are useful for reading ladder type bar code symbolsdisposed on bottom-, front-, land right-facing surfaces.

[0153] FIGS. 3F1 and 3F2 illustrate the fourth group GH4 of laser beamfolding mirrors of the first laser scanning station (HST1), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate eight different horizontal laser scanningplanes that project from the front side of the bottom-scanning window 16diagonally outward and upward above the back side of the bottom-scanningwindow 16 as shown. Note that the first laser scanning station HST1utilizes mirrors MH4 and MH5 (and not MH6) of group GH4 to produce eightdifferent scan planes therefrom. These scanning planes are useful forreading picket-fence type bar code symbols disposed on bottom- andfront-facing surfaces.

[0154] The position and orientation of each beam folding mirror employedat scanning station HST1 relative to a global coordinate referencesystem is specified by a set of position vectors pointing from the fromthe origin of this global coordinate reference system to the vertices ofeach such beam folding mirror element (i.e. light reflective surfacepatch). The x,y,z coordinates of these vertex-specifying vectors as setforth above in Table I specify the perimetrical boundaries of these beamfolding mirrors employed in the scanning system of the illustrativeembodiment.

[0155] Second Laser Scanning Station HST2

[0156] As shown in FIGS. 2A, 2B and 4A-4E2, the second laser scanningstation (HST2) includes four groups of laser beam folding mirrors (GH4,GH5, GH6, and GH7) which are arranged about the first rotating polygonalmirror PM1, and cooperate with the four scanning facets of the firstrotating polygonal mirror PM1 so as to generate and project fourdifferent groups of laser scanning planes (with 20 total scanning planesin the four groups) through the bottom-scanning window 16. Theintersection of the four groups of laser scanning planes (with 20 totalscanning planes in the four groups) on the bottom-scanning window 16 isshown in FIG. 4A. The twenty laser scanning planes (of these four groupsprojected through the bottom-scanning window 16) can be classified aseither vertical scanning planes or horizontal scanning planes as definedabove.

[0157] FIGS. 4B1 and 4B2 illustrate the first group (GH4) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate eight different horizontal laser scanningplanes that project from the front side of the bottom-scanning window 16diagonally outward and upward above the back side of the bottom-scanningwindow 16 as shown. Note that the second laser scanning station HST2utilizes mirrors MH5 and MH6 (and not MH4) of group GH4 to produce eightdifferent scan planes therefrom. These scanning planes are useful forreading picket-fence type bar code symbols disposed on bottom- andfront-facing surfaces.

[0158] FIGS. 4C1 and 4C2 illustrate the second group (GH5) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the left front corner of the bottom-scanningwindow 16 diagonally outward and upward above the back right side andback right corner of the bottom-scanning window 16 as shown. Thesescanning planes are useful for reading ladder type bar code symbolsdisposed on bottom-, front-, and left-facing surfaces.

[0159] FIGS. 4D1 and 4D2 illustrate the third group (GH6) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different horizontal laser scanningplanes that project from the left side of the bottom-scanning window 16diagonally outward and upward above the right side of thebottom-scanning window 16 as shown. These scanning planes are useful forreading picket-fence type bar code symbols disposed on bottom- andleft-facing surfaces.

[0160] FIGS. 4E1 and 4E2 illustrate the fourth group (GH7) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the left back corner of the bottom-scanningwindow 16 diagonally outward and upward above the front right side andfront right corner of the bottom-scanning window 16 as shown. Thesescanning planes are useful for reading ladder type bar code symbolsdisposed on bottom-, back-, and left-facing surfaces.

[0161] The position and orientation of each beam folding mirror employedat scanning station HST2 relative to a global coordinate referencesystem is specified by a set of position vectors pointing from the fromthe origin of this global coordinate reference system to the vertices ofeach such beam folding mirror element (i.e. light reflective surfacepatch). The x,y,z coordinates of these vertex-specifying vectors as setforth above in Table I specify the perimetrical boundaries of these beamfolding mirrors employed in the scanning system of the illustrativeembodiment.

[0162] As shown in FIG. 4F, the vertical scanning planes that projectfrom the bottom-scanning window 16 include 4 groups (namely, GH1, GH3,GH5 and GH7). Groups GH1 and GH5 project from opposing portions (e.g.,the back-right and front-left corners of the window 16) of thebottom-scanning window 16, and groups GH3 and GH7 project from opposingportions (e.g., front-right and back-left corners of the window 16) ofthe bottom-scanning window. Note that groups GH1 and GH5 aresubstantially co-planar (i.e., quasi co-planar) and groups GH3 and GH7are substantially co-planar (i.e., quasi co-planar), while groups GH1and GH5 are substantially orthogonal (i.e., quasi-orthogonal) to groupsGH3 and GH7, respectively, as shown.

[0163] Third Laser Scanning Station VST1

[0164] As shown in FIGS. 2D, 2E and 5A-5P2, the third laser scanningstation (VST1) includes fourteen groups of laser beam folding mirrors(GV1, GV2, GV3 . . . GV14) which are arranged about the second rotatingpolygonal mirror PM2, and cooperate with the four scanning facets of thesecond rotating polygonal mirror PM2 so as to generate and projectfourteen different groups of laser scanning planes (with 28 totalscanning planes in the fourteen groups) through the side-scanning window18. The intersection of the fourteen groups of laser scanning planes(with 28 total scanning planes in the fourteen groups) on theside-scanning window 18 is shown in FIG. 5A. The twenty-eight laserscanning planes (of these fourteen groups projected through theside-scanning window 18) can be classified as either vertical scanningplanes or horizontal scanning planes, which can be defined as follows.

[0165] As shown in FIG. 5B1 and 5B2, a scanning plane has acharacteristic direction of propagation D_(p) and a normal directionSP_(N), which define a direction O that is orthogonal thereto (e.g.,O=D_(p)×SP_(N)). A horizontal scanning plane is a scanning plane whereinthe angle φ between the direction O and the plane defined by thebottom-scanning window 16 is in the range between 0 and 45 degrees (andpreferably in the range between 0 and 20 degrees, and more preferably inthe range between 0 and 10 degrees). An exemplary horizontal scanningplane projected from the side-scanning window 18 is shown in FIG. 5B1. Avertical scanning plane is a scanning plane wherein the angle φ betweenthe direction O and the plane defined by the bottom-scanning window 16is in the range between 45 and 90 degrees (and preferably in the rangebetween 70 and 90 degrees, and more preferably in the range between 80and 90 degrees). An exemplary vertical scanning plane projected from theside-scanning window 18 is shown in FIG. 5B2.

[0166] FIGS. 5C1 and 5C2 illustrate the first group (GV1) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low-elevation (LE) scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differentvertical laser scanning planes that project from the left side of theside-scanning window 18 diagonally down and out across thebottom-scanning window 16 above the front right corner of thebottom-scanning window 16 as shown. These scanning planes are useful forreading ladder type bar code symbols disposed on left- and back-facingsurfaces.

[0167] FIGS. 5D1 and 5D2 illustrate the second group (GV2) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low-elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differentvertical laser scanning planes that project from the top left corner ofthe side-scanning window 18 downward toward the bottom-scanning window16 substantially along the left side of the bottom-scanning window 16 asshown. These scanning planes are useful for reading ladder type bar codesymbols disposed on top- and back-facing surfaces.

[0168] FIGS. 5E1 and 5E2 illustrate the third group (GV3) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low-elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top left quadrantof the side-scanning window 18 diagonally down across thebottom-scanning window 16 as shown. These scanning planes are useful forreading picket-fence type bar code symbols disposed on back- andtop-facing surfaces.

[0169] FIGS. 5F1 and 5F2 illustrate the fourth group (GV4) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top rightquadrant of the side-scanning window 18 diagonally down across thebottom-scanning window 16 as shown. These scanning planes are useful forreading picket-fence type bar code symbols disposed on back- andtop-facing surfaces.

[0170] FIGS. 5G1 and 5G2 illustrate the fifth group (GV5) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low-elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differentvertical laser scanning planes that project from the top right corner ofthe side-scanning window 18 downward toward the bottom-scanning window16 substantially along the right side of the bottom-scanning window 16as shown. These scanning planes are useful for reading ladder type barcode symbols disposed on top- and back-facing surfaces.

[0171] FIGS. 5H1 and 5H2 illustrate the sixth group (GV6) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differentvertical laser scanning planes that project from the right side of theside-scanning window 18 diagonally out across the bottom-scanning window16 above the front left corner of the bottom-scanning window 16 asshown. These scanning planes are useful for reading ladder type bar codesymbols disposed on right- and back-facing surfaces.

[0172] FIGS. 5I1 and 5I2 illustrate the seventh group (GV7) of laserbeam folding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top left quadrantof the side-scanning window 18 outwardly across the bottom-scanningwindow 16 (substantially parallel to the bottom-scanning window 16) asshown. These scanning planes are useful for reading picket-fence typebar code symbols disposed on back- and left-facing surfaces.

[0173] FIGS. 5J1 and 5J2 illustrate the eight group (GV8) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the left side of theside-scanning window 18 outwardly across the bottom-scanning window 16(substantially parallel to the bottom-scanning window 16) as shown. Inthe illustrative embodiment, the characteristic direction of propagationof such scanning planes has a non-vertical component (i.e., componentsin the plane parallel to the bottom-scanning window 16) whoseorientation relative to the normal of the side-scanning window 18 isgreater than 35 degrees. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on back- and left-facingsurfaces (including those surfaces whose normals are substantiallyoffset from the normal of the side-scanning window).

[0174] FIGS. 5K1 and 5K2 illustrate the ninth group (GV9) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly and downward across thebottom-scanning window 16 as shown. These scanning planes are useful forreading picket-fence type bar code symbols disposed on back-facingsurfaces.

[0175] FIGS. 5L1 and 5L2 illustrate the tenth group (GV10) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly and sharply downward across thebottom-scanning window 16 as shown. These scanning planes are useful forreading picket-fence type bar code symbols disposed on top- andback-facing surfaces.

[0176] FIGS. 5M1 and 5M2 illustrate the eleventh group (GV11) of laserbeam folding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly and sharply downward across thebottom-scanning window 16 as shown. These scanning planes are useful forreading picket-fence type bar code symbols disposed on top- andback-facing surfaces.

[0177] FIGS. 5N1 and 5N2 illustrate the twelfth group (GV12) of laserbeam folding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly across the bottom-scanningwindow 16 (substantially parallel to the bottom-scanning window 16) asshown. These scanning planes are useful for reading picket-fence typebar code symbols disposed on back-facing surfaces.

[0178] FIGS. 5O1 and 5O2 illustrate the thirteenth group (GV13) of laserbeam folding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G 1) so as to generate twodifferent horizontal laser scanning planes that project from the rightside of the side-scanning window 18 outwardly across the bottom-scanningwindow 16 (substantially parallel to the bottom-scanning window 16) asshown. In the illustrative embodiment, the characteristic direction ofpropagation of such scanning planes has a non-vertical component (i.e.,components in the plane parallel to the bottom-scanning window 16) whoseorientation relative to the normal of the side-scanning window 18 isgreater than 35 degrees. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on back- and right-facingsurfaces (including those surfaces whose normals are substantiallyoffset from the normal of the side-scanning window).

[0179] FIGS. 5P1 and 5P2 illustrate the fourteenth group (GV14) of laserbeam folding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top rightquadrant of the side-scanning window 18 outwardly across thebottom-scanning window 16 (substantially parallel to the bottom-scanningwindow 16) as shown. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on back- and right-facingsurfaces.

[0180] The position and orientation of each beam folding mirror employedat scanning station VST1 relative to a global coordinate referencesystem is specified by a set of position vectors pointing from the fromthe origin of this global coordinate reference system to the vertices ofeach such beam folding mirror element (i.e. light reflective surfacepatch). The x,y,z coordinates of these vertex-specifying vectors as setforth above in Table III specifies the perimetrical boundaries of thesebeam folding mirrors employed in the scanning system of the illustrativeembodiment.

[0181] In the illustrative embodiment of the present invention, thelaser beam folding mirrors associated with scanning stations HST1, HST2and VST1 are physically supported utilizing one or more mirror supportplatforms, formed with the scanner housing. Preferably, these mirrormounting support structures, as well as the components of the scanninghousing are made from a high-impact plastic using injection moldingtechniques well known in the art.

[0182] In the scanning system of the present invention, the principalfunction of each facet on the first and second rotating polygonalmirrors PM1 and PM2 is to deflect an incident laser beam along aparticular path in 3-D space in order to generate a correspondingscanning plane within the 3-D laser scanning volume produced by thelaser scanning system hereof. Collectively, the complex of laserscanning planes produced by the plurality of facets in cooperation withthe three laser beam production modules of HST1, HST2 and VST1 createsan omni-directional scanning pattern within the highly-defined 3-Dscanning volume of the scanning system between the space occupied by thebottom and side-scanning windows of the system. As shown in theexemplary timing scheme of FIG. 6, the bioptical laser scanner of theillustrative embodiment cyclically generates a complex omni-directional3-D laser scanning pattern from both the bottom and side-scanningwindows 16 and 18 thereof during the revolutions of the scanningpolygonal mirrors PM1 and PM2. In this exemplary timing scheme, foursets of scan plane groups (4*[GH1 . . . GH7]) are produced by stationsHST1 and HST2 during each revolution of the polygonal mirror PM1concurrently with a two sets of scan plane groups (2*[GV1 . . . GV14])produced by station VST1 during a single revolution of the polygonalmirror PM2. This complex omnidirectional scanning pattern is graphicallyillustrated in FIGS. 3A through 5P2. The 3-D laser scanning pattern ofthe illustrative embodiment consists of 68 different laser scanningplanes, which cooperate in order to generate a plurality ofquasi-orthogonal laser scanning patterns within the 3-D scanning volumeof the system, thereby enabling true omnidirectional scanning of barcode symbols.

[0183] In each laser scanning station (HST1, HST2, and VST1) of theillustrative embodiment, a laser beam production module produces a laserbeam that is directed at the point of incidence on the facets of thefirst or second rotating polygonal mirrors at the pre-specified angle ofincidence. Preferably, such laser beam production modules comprises avisible laser diode (VLD) and possibly an aspheric collimating lenssupported within the bore of a housing mounted upon the optical bench ofthe module housing.

[0184] In the illustrative embodiment described above, the pre-specifiedangle of incidence of the laser beams produced by the laser beamproduction modules of the laser scanning stations HST1 and HST2 areoffset from the rotational axis of the polygonal mirror PM1 along adirection perpendicular to the rotational axis as shown in FIG. 2H. Suchoffset provides for spatial overlap in the scanning pattern of lightbeams produced from the polygonal mirror PM1 by these laser beamproduction modules. In the illustrative embodiment, the offset betweenthe rotational axis of the rotating polygonal mirror PM1 and theincident directions of the scanning beams SB1 and SB2, respectively, isapproximately 5 mm. Such spatial overlap can be exploited such that theoverlapping rays are incident on at least one common mirror (mh5 in theillustrative embodiment) to provide a dense scanning pattern projectingtherefrom. In the illustrative embodiment, a dense pattern of horizontalplanes (groups GH4) is projected from the front side of the bottomwindow as is graphically depicted in FIGS. 3F1, 3F2 and 4B1 and 4B2.

[0185] Light Collection for the 3 Scanning Stations

[0186] When a bar code symbol is scanned by any one of the laserscanning planes projected from the bottom-scanning window 16 (by eitherthe first or second laser scanning stations HST1, HST2), or by any oneof the laser scanning planes projected from the side-scanning window 18by the third laser scanning station VST1, the incident laser lightscanned across the object is intensity modulated by the absorptiveproperties of the scanned object and scattered according to Lambert'sLaw (for diffuse reflective surfaces). A portion of this laser light isreflected back along the outgoing ray (optical) path, off the same groupof beam folding mirrors employed during the corresponding laser beamgeneration process, and thereafter is incident on the same scanningfacet (of the first or second rotating polygonal mirror) that generatedthe corresponding scanning plane only a short time before. The scanningfacet directs the returning reflected laser light towards a lightcollecting optical element (e.g., parabolic mirror structure) of therespective laser scanning station, which collects the returning lightand focuses these collected light rays onto a photodetector, which maybe disposed on a planar surface beneath the respective scanning polygon(as shown in FIGS. 2C1 and 2C2), or which may be disposed on a planarsurface above the respective scanning polygon (as shown in FIG. 2F).FIGS. 2C1 and 2C2 depict the light collection optical elements LC_(HST1)and LC_(HST2), e.g., parabolic mirrors, and photodetectors PD_(HST1) andPD_(HST2) for the two laser scanning stations HST1 and HST2,respectively. FIG. 2F depicts the light collection optical elementsLC_(VST1, e.g., parabolic mirror, and photodetector PD)_(VST1 for the third laser scanning station VST1. The electrical signal produced by the photodetector for the respective laser scanning stations is supplied to analog/digital signal processing circuitry, associated with the respective laser scanning stations, that process analog land digital scan data signals derived therefrom to perform bar code symbol reading operations.)

[0187] As shown in FIGS. 1A, the bottom and side-scanning windows 16 and18 have light transmission apertures of substantially planar extent. Inorder to seal off the optical components of the scanning system fromdust, moisture and the like, the scanning windows 16 and 18, arepreferably fabricated from a high impact plastic material and installedover their corresponding light transmission apertures using a rubbergasket and conventional mounting techniques. In the illustrativeembodiment, each scanning window 16 and 18 preferably hasspectrally-selective light transmission characteristics which, inconjunction with a spectrally-selective filters installed before eachphotodetector within the housing, forms a narrow-band spectral filteringsubsystem that performs two different functions. The first function ofthe narrow-band spectral filtering subsystem is to transmit only theoptical wavelengths in the red region of the visible spectrum in makesthe internal optical components less visible and thus remarkablyimproves the external appearance of the bioptical laser scanning system.This feature also makes the bioptical laser scanner less intimidating tocustomers at point-of-sale (POS) stations where it may be used. Thesecond function of the narrow-band spectral filtering subsystem is totransmit to the photodetector for detection, only the narrow band ofspectral components comprising the outgoing laser beam produced by theassociated laser beam production module. Details regarding this opticalfiltering subsystem are disclosed in copending application Ser. No.08/439,224, entitled “Laser Bar Code Symbol Scanner Employing OpticalFiltering With Narrow Band-Pass Characteristics and Spatially SeparatedOptical Filter Elements” filed on May 11, 1995, which is incorporatedherein by reference in its entirety.

[0188] Electrical Subsystem

[0189] In the illustrative embodiment of the present invention, thebioptical laser scanning system 1 comprises a number of systemcomponents as shown in the system diagram of FIG. 7, including:photodetectors (e.g. a silicon photocell) for detection of optical scandata signals generated by the respective laser scanning stations; analogsignal processing circuitry for processing (e.g., preamplification,bandpass filtering, and A/D conversion) analog scan data signals,digitizing circuitry for converting the digital scan data signal D₂,associated with each scanned bar code symbol, into a correspondingsequence of digital words (i.e. a sequence of digital count values) D₃,and bar code symbol decoding circuitry that receives the digital wordsequences D₃ produced from the digitizing circuit, and subject it to oneor more bar code symbol decoding algorithms in order to determine whichbar code symbol is indicated (i.e. represented) by the digital wordsequence D₃.

[0190] As described above, during laser scanning operations, the opticalscan data signal Do focused onto the photodetectors 45A 45B, and 45C isproduced by light rays associated with a diffracted laser beam beingscanned across a light reflective surface (e.g. the bars and spaces of abar code symbol) and scattering thereof, whereupon the polarizationstate distribution of the scattered light rays is typically altered whenthe scanned surface exhibits diffuse reflective characteristics.Thereafter, a portion of the scattered light rays are reflected alongthe same outgoing light ray paths toward the facet which produced thescanned laser beam. These reflected light rays are collected by thescanning facet and ultimately focused onto the photodetector by itsparabolic light reflecting mirror. The function of each photodetector isto detect variations in the amplitude (i.e. intensity) of optical scandata signal D₀, and produce in response thereto an electrical analogscan data signal D_(I) which corresponds to such intensity variations.When a photodetector with suitable light sensitivity characteristics isused, the amplitude variations of electrical analog scan data signalD_(I) will linearly correspond to light reflection characteristics ofthe scanned surface (e.g. the scanned bar code symbol). The function ofthe analog signal processing circuitry is to amplify and band-passfilter the electrical analog scan data signal D₁, in order to improvethe SNR of the analog signal, and convert the analog signal into digitalform (e.g., a pulse train with transitions between high and low logiclevels). In practice, this analog to digital conversion is athresholding function which converts the electrical analog scan datasignal D₁ into a corresponding digital scan data signal D₂ having firstand second (i.e. binary) signal levels which correspond to the bars andspaces of the bar modulated type signal as the first and second signallevels vary in proportion to the width of bars and spaces in the scannedbar code symbol.

[0191] The digitizing circuitry converts the digital scan data signalD₂, associated with each scanned bar code symbol, into a correspondingsequence of digital words (i.e. a sequence of digital count values) D₃.Notably, in the digital word sequence D₃, each digital word representsthe time length associated with each first or second signal level in thecorresponding digital scan data signal D₂. Preferably, these digitalcount values are in a suitable digital format for use in carrying outvarious symbol decoding operations which, like the scanning pattern andvolume of the present invention, will be determined primarily by theparticular scanning application at hand. Reference is made to U.S. Pat.No. 5,343,027 to Knowles, incorporated herein by reference, as itprovides technical details regarding the design and construction ofmicroelectronic digitizing circuits suitable for use in the laserscanner of the present invention.

[0192] The bar code symbol decoding circuitry receive each digital wordsequence D₃ produced from the digitizing circuit, and subject it to oneor more bar code symbol decoding algorithms in order to determine whichbar code symbol is indicated (i.e. represented) by the digital wordsequence D₃, originally derived from corresponding scan data signal D₁detected by the photodetector associated therewith. In more generalscanning applications, the function of the programmed decode computer isto receive each digital word sequence D₃ produced from the digitizingcircuit, and subject it to one or more pattern recognition algorithms(e.g. character recognition algorithms) in order to determine whichpattern is indicated by the digital word sequence D₃. In bar code symbolreading applications, in which scanned code symbols can be any one of anumber of symbologies, a bar code symbol decoding algorithm withauto-discrimination capabilities can be used in a manner known in theart.

[0193] As shown in FIG. 7, the system includes a programmedmicroprocessor 61 with a system bus and associated program and datastorage memory, for controlling the system operation of the biopticallaser scanner and performing other auxiliary functions and for receivingbar code symbol character data (provided by the bar code symbol decodingcircuitry); a data transmission subsystem for interfacing with andtransmitting symbol character data and other information to hostcomputer system (e.g. central computer, cash register, etc.) over acommunication link therebetween; and an input/output interface forproviding drive signals to an audio-transducer and/or LED-based visualindicators used to signal successful symbol reading operations to usersand the like, for providing user input via interaction with a keypad,and for interfacing with a plurality of accessory devices (such as anexternal handheld scanner that transmits bar code symbol character datato the bioptical laser scanning system, a display device, a weightscale, a magnetic card reader and/or a coupon printer as shown). Inaddition, the input-output interface may provide a port that enables anexternal handheld scanner to transmit sequences of digital words D₃(i.e. a sequence of digital count values) generated therein to thebioptical laser scanning system for bar code symbol decoding operations.Details of such an interface port are described in U.S. Pat. No.5,686,717 to Knowles et al., commonly assigned to the assignee of thepresent invention, herein incorporated by reference in its entirety.

[0194] The communication link between the data transmission subsystemand the host system may be a wireless data link (such as an infra-redlink, Bluetooth RF link or IEEE 802.11a or 802.11b RF link) or wiredserial data link (such as keyboard wedge link—for example supportingXT-, AT- and PS/2-style keyboard protocols, an RS-232 link, USB link, aFirewire (or IEEE 11394) link, an RS-422 link, and RS-485 link), a wiredparallel data bus, or other common wired interface links (such as anOCIA link, IBM 46XX link, Light Pen Emulation link, LTPN link).similarly, the input/output interface between the external handheldscanner and the bioptical laser scanning system may support a wirelessdata link (such as an infra-red link, Bluetooth RF link or IEEE 802.11aor 802.11b RF link) or wired serial data link (such as keyboard wedgelink—for example supporting XT-, AT- and PS/2-style keyboard protocols,an RS-232 link, USB ilink, a Firewire (or IEEE 1394) link, an RS-422link, and RS-485 link), a wired parallel data bus, or other common wiredinterface links (such as an OCIA link, IBM 46XX link, Light PenEmulation link, LTPN link).

[0195] The microprocessor also produces motor control signals, and lasercontrol signals during system operation. These control signals as wellas a 120 Volt, 60 Hz line voltage signal from an external power source(such as a standard power distribution circuit) are received as input bya power regulation circuit, which produces as output, (1) laser sourceenable signals to drive VLDs 153A, 153B, 153C, and 153D, respectively,and (2) motor enable signals in order to drive the two motors (motor 1and motor 2) that cause rotation of the first and second rotatingpolygonal mirrors, respectively.

[0196] In some scanning applications, where omni-directional scanningcannot be ensured at all regions within a pre-specified scanning volume,it may be useful to use scan data produced either (i) from the samelaser scanning plane reproduced many times over a very short timeduration while the code symbol is being scanned therethrough, or (ii)from several different scanning planes spatially contiguous within apre-specified portion of the scanning volume. In the first instance, ifthe bar code symbol is moved through a partial region of the scanningvolume, a number of partial scan data signal fragments associated withthe moved bar code symbol can be acquired by a particular scanning planebeing cyclically generated over an ultra-short period of time (e.g. 1-3milliseconds), thereby providing sufficient scan data to read the barcode symbol. In the second instance, if the bar code symbol is withinthe scanning volume, a number of partial scan data signal fragmentsassociated with the bar code symbol can be acquired by several differentscanning planes being simultaneously generated by the three laserscanning stations of the system hereof, thereby providing sufficientscan data to read the bar code symbol, that is, provided such scan datacan be identified and collectively gathered at a particular decodeprocessor for symbol decoding operations.

[0197] In order to allow the bioptical scanning system of the presentinvention to use symbol decoding algorithms that operate upon partialscan data signal fragments, as described above, a synchronizing signalcan be used to identify a set of digital word sequences D₃, (i.e.{D_(S)}), associated with a set of time-sequentially generated laserscanning beams produced by a particular facet on the first and secondrotating polygonal mirrors. In such applications, each set of digitalword sequences can be used to decode a partially scanned code symbol andproduce symbol character data representative of the scanned code symbol.In code symbol reading applications where complete scan data signals areused to decode scanned code symbols, the synchronizing signal describedabove need not be used, as the digital word sequence D₃ corresponding tothe completely scanned bar code symbol is sufficient to carry out symboldecoding operations using conventional symbol decoding algorithms knownin the art.

[0198] The synchronizing signal can be derived from a position sensor(such as a hall sensor), integrated into the rotating shaft (or otherportion) of the rotating polygonal mirror, that generates an electricalsignal when the rotating polygonal mirror reaches a predetermined point(such as a start-of-scan position) in its rotation. Alternatively, suchsynchronization may be derived from a position indicating opticalelement (e.g., mirror or lens), which is preferably mounted adjacent (ornear) the perimeter of one of the light folding mirrors, such that theposition indicating optical element is illuminated by the scanning beamwhen the rotating polygonal mirror reaches a predetermined point (suchas a start-of-scan position) in its rotation. The position indicatingoptical element may be a mirror that directs the illumination of thescanning beam incident thereon to a position indicating optical detector(which generates an electrical signal whose amplitude corresponds to theintensity of light incident thereon). Alternatively, the positionindicating optical element may be a light collecting lens that isoperably coupled to a light guide (such as a fiber optic bundle) thatdirects the illumination of the scanning beam incident thereon to aposition indicating optical detector (which generates an electricalsignal whose amplitude corresponds to the intensity of light incidentthereon).

[0199] As each synchronizing pulse in the synchronizing signal issynchronous with a “reference” point on the respective rotating mirror,the symbol decoding circuitry provided with this periodic signal canreadily “link up” or relate, on a real-time basis, such partial scandata signal fragments with the particular facet on the respectiverotating polygonal mirror that generated the partial scan data fragment.By producing both a scan data signal and a synchronizing signal asdescribed above, the bioptical laser scanning system of the presentinvention can readily carry out a diverse repertoire of symbol decodingprocesses which use partial scan data signal fragments during the symbolreading process.

[0200] Modifications

[0201] The bioptical laser scanning system of the present invention canbe modified in various ways. For example, more (or less) groups of beamfolding mirrors can be used in each laser scanning station within thesystem and/or more or less facets can be used for the rotating polygonalmirrors, Such modifications will add (or remove) scanning planes fromthe system.

[0202] Also more or less laser scanning stations might be employedwithin the system. Such modifications might be practiced in order toprovide an omnidirectional laser scanning pattern having scanningperformance characteristics optimized for a specialized scanningapplication.

[0203] While the second rotating polygonal mirror of the illustrativeembodiment employs facets having low and high elevation anglecharacteristics, it is understood that it might be desirable inparticular applications to use scanning facets with differentcharacteristics (such as varying angular reflection characteristics) soas to enable a compact scanner design in a particular application.

[0204] Also, it is contemplated that each laser scanning station may nothave its own laser source (e.g., VLD). More specifically, as is wellknown in the scanning art, the laser light produced by a laser source(VLD) may be split into multiple beams (with a beam splitter) anddirected to multiple laser scanning stations with mirrors, a light pipeor other light directing optical element.

[0205] While the various embodiments of the bioptical laser scannerhereof have been described in connection with linear (1-D) bar codesymbol scanning applications, it should be clear, however, that thescanning apparatus and methods of the present invention are equallysuited for scanning 2-D bar code symbols, as well as alphanumericcharacters (e.g. textual information) in optical character recognition(OCR) applications, as well as scanning graphical images in graphicalscanning arts.

[0206] Improved Scan Signal Processing

[0207] In any laser scanning system (including the various embodimentsof the bioptical laser scanner described above), the primary function ofthe laser scanning mechanism is to produce a laser scanning field (orvolume) in which bar code symbols can be scanned in a reliable manner.In such systems, the speed of the laser beam spot (or cross-section)along the extent of the scanned laser beam will vary over the depth ofthe scanning range of the system. The further the laser beam spot isaway from the laser scanning mechanism, the greater the laser beam spotspeed with be, based on well known principles of physics. A usefulmeasure of such beam spot speed variation is given by the ratio of (i)the maximum laser beam spot speed within the scanning field of thesystem, to (ii) the minimum laser beam spot speed in the scanningsystem. Hereinafter, this spot speed variation measure shall be referredto as the “Max/Min Beam Spot Speed Ratio” of a laser scanning system.

[0208] The substrate, usually paper, on which a bar code is printedreflects a signal of varying power when scanned with a focused laserbeam within a given focal zone in the system. The laser light energyreflected (i.e. scattered) off the scanned code symbol is directed ontoa photodetector by way of light collection and focusing optics. Thephotodetector converts these optical signals into correspondingelectrical signals. The signal components produced by scanning the barcode substrate are unwanted and therefore are described as noise. Sincethe substrate is usually paper, consisting of fibers having a randomspatial structure, such unwanted noise signals are commonly referred toas paper or substrate noise. A signal derived from the output of thephotodetector (in analog or digital form) is referred to as a scan datasignal S_(analog) comprising the desired bar code signal component aswell as the paper noise components.

[0209] As a bar code is scanned within a focal zone disposed furtheraway from the scanner, the scan data signal is increasingly compressedon the time-domain by virtue of the fact that the laser beam speedincreases as a function of distance away from the laser scanningmechanism. In accordance with Fourier Analysis principles, compressionof the scan data signal (including its noise components) represented onthe time-domain results in an increase in or shift of power to thehigher spectral components of the scan data signal represented on thefrequency-domain. Thus, the frequency spectra of the scan data signal(including its noise components) undergoes a positive frequency shift asthe corresponding bar code symbol is scanned further away from the laserscanning system. This phenomenon is graphically illustrated in theanalog scan data signal of FIGS. 8A and 8B.

[0210] When scanning bar code symbols in a multi-focal zone laserscanning system, filters and signal thresholding devices are useful forrejecting noise components in the scan data signal. However, suchdevices also limit the scan resolution of the system, potentiallyrendering the system incapable of reading low contrast and highresolution bar code symbols on surfaces placed in the scanning field.Thus, it is imperative that the bandwidth of the system be sufficient tosupport the spectral components of scan data signals at different focalzones of the system and to support the scanning of the desiredresolution of bar code symbols on surfaces placed in the scanning field.

[0211] In accordance with teachings of the present invention, a laserscanning system (such as the bioptical laser scanning system of thepresent invention as described above) includes a multi-path scan datasignal processor having multiple signal processing paths. Each signalprocessing path processes the same data signal (which is derived fromthe output of a photodetector) to detect bar code symbols therein andgenerate data representing the bar code symbols. And each signalprocessing path has different operational characteristics (such aslow-pass filter cutoff frequencies, amplifier gain characteristics,and/or positive and negative signal thresholds). The varying operationalcharacteristics of the paths are optimized to provide different signalprocessing functions (e.g., minimize paper noise, or maximize the scanresolution of the system). The data signal derived from laser scanningis supplied to each path of the multi-path scan data processor, where itis processed (preferably in parallel) to identify signal leveltransitions therein. A digital scan data signal that encodes such signallevel transitions is provided to digitizing circuitry, which convertsthe digital scan data signal into a corresponding sequence of digitalwords (i.e. a sequence of digital count values) suitable for bar codesymbol decoding as described above.

[0212] By virtue of the present invention, it is now possible toidentify signal level transitions in the scan data signal over a diverserange of operating conditions (e.g., operating conditions where papernoise is present in addition to operating conditions requiring highresolution scanning, such as the reading of low contrast or highresolution bar code symbols), which enables more reliable bar codereading over such diverse operating conditions. These and otheradvantages of the present invention will become apparent hereinafter.

[0213] Analog Scan Data Signal Processor of the Illustrative Embodimentof the Present Invention

[0214] As shown in FIG. 9, a multi-path scan data signal processor 901according to the present invention comprises a number of subcomponents,namely: signal conditioning circuitry 903 operably coupled between aphotodetector 902 and a plurality of signal processing paths (two shownas path A and path B) that process the output of the signal conditioningcircuitry in parallel. Each signal processing path includes: a firstderivative signal generation circuit 904 having a differentiator, lowpass filter and amplifier therein; a second derivative signal generationcircuit 906 having a differentiator therein; a first derivative signalthreshold-level generation circuit 905; and a zero crossing detector907, data gate 908, and binary-type A/D signal conversion circuitry 909.

[0215] The signal conditioning circuitry 903 operates to smooth out orotherwise filter the scan data signal produced by the photodetector 902to remove unwanted noise components therein, and possibly amplify suchsignal. An illustrative implementation of such signal conditioningcircuitry is described below with respect to FIG. 11. The output of thesignal conditioning circuitry 903 is provided to the plurality of signalprocessing paths (two shown as path A and path B) that process theoutput of the signal conditioning circuitry 903 in parallel.

[0216] The first derivative signal generation circuitry 904 in eachrespective path (labeled 904-A and 904-B in as shown) includes adifferentiator, low pass filter and amplifier that generate a signalapproximating the first derivative of the analog scan data signal (withunwanted noise components removed). The low pass filter may beimplemented with passive elements (such as resistors, capacitors andinductors) or may be implemented with active elements (such as anoperational amplifier). Preferably, the low-pass filter implements oneof a Butterworth-type, Chebsychev-type, MFTD-type, or elliptical-typelow pass filtering transfer function, which are Cwell known in thefiltering art. Details of the design of such filters is set forth in thebook entitled “Electrical Filter Design Handbook,” Third Edition, by A.Williams et al., McGraw-Hill, X996, herein incorporated by reference inits entirety. An illustrative implementation of the first derivativesignal generation circuitry 904 for two different paths is describedbelow with respect to FIG. 12.

[0217] The “first derivative signal” is supplied to second derivativesignal generation circuit 906 and to first derivative thresholdcircuitry 905 in the respective path. The second derivative signalgeneration circuitry in each respective path (labeled 906-A and 906-B asshown) includes a differentiator that generates a signal approximatingthe second derivative of the scan data signal (with unwanted noisecomponents removed). An example of the second derivative signalgeneration circuitry is described below with respect to FIG. 13.

[0218] The “second derivative signal” is supplied to a zero crossingdetector 907 that produces output signal(s) (“zero crossing signal”)identifying zero crossings in the second derivative signal. Anillustrative implementation of the zero crossing detector in eachrespective path (labeled 907-A and 907-B) is described below withrespect to FIG. 15.

[0219] The first derivative threshold circuitry in each respective path(labeled 905-A and 905-B) operates as a positive and negative peakdetector to provide output signals that indicate the approximate timeperiods when the positive and negative peaks of the first derivativesignal if provided thereto exceed predetermined thresholds (i.e., apositive peak level PPL and a negative peak level NPL). An illustrativeimplementation of such first derivative threshold circuitry 905 for thetwo different paths is described below with respect to FIG. 14.

[0220] In the absence of noise, the occurrence of each second derivativezero-crossing indicates that the “first derivative signal” is undergoinga (positive or negative) peak which corresponds to the point in the scandata signal where a signal level transition (e.g., indicative of atransition between a space and a bar in a bar code symbol) has occurred.However, in the real-world, noise is notorious for producing falsezero-crossing detections within the second derivative zero-crossingdetection circuit. To reduce the number of “falsely detected”zero-crossings produced by noise, data gating circuit 908 is provided,which functions to gate to the binary-type A/D signal conversioncircuitry 909, only detected second derivative zero-crossings whichoccur substantially concurrent to a positive or negative peak detectedin the “first derivative signal” (as identified by the outputs signalsof the first derivative threshold circuitry 905). An example of the datagate circuitry and binary-type A/D signal conversion circuitry isdescribed below with respect to FIG. 16.

[0221] The output of the binary-type A/D conversion circuitry 909 is adigital scan data signal D₂ having first and second (i.e. binary) signallevels which correspond to the bars and spaces of the bar code symbolbeing scanned. Thus, the digital scan data signal D₂ appears as apulse-width modulated type signal as the first and second signal levelsvary in proportion to the width of bars and spaces in the scanned barcode symbol.

[0222] The digital scan data signal D₂ is supplied to digitizingcircuitry, which converts the digital scan data signal D₂, associatedwith each scanned bar code symbol, into a corresponding sequence ofdigital words (i.e. a sequence of digital count values) D₃. Notably, inthe digital sword sequence D₃, each digital word represents the timelength associated with each first or second signal level in thecorresponding digital scan data signal D₂. Preferably, these digitalcount values are in a suitable digital format for use in carrying outvarious symbol decoding operations which, like the scanning pattern andvolume of the present invention, will be determined primarily by theparticular scanning application at hand. Reference is made to U.S. Pat.No. 5,343,027 to Knowles, incorporated herein by reference, as itprovides technical details regarding the design and construction ofmicroelectronic digitizing circuits suitable for use in the laserscanner of the present invention.

[0223] Bar code symbol decoding circuitry (which is typicallyimplemented with a programmed microprocessor/microcontroller) receiveeach digital word sequence D₃ produced from the digitizing circuit, andsubject it to one or more bar code symbol decoding algorithms in orderto determine which bar code symbol is indicated (i.e. represented) bythe digital word sequence D₃, originally derived from corresponding scandata signal DI detected by the photodetector associated therewith.

[0224] The operation of the multi-path scan data signal processor 901 isillustrated by the signal diagrams of FIGS. 10A through 10I. FIGS. 10Aand 10B depict the signal produced at the output of the photodetector902 as the laser scanning beam scans across a bar code symbol. FIG. 10Cdepicts the output signal produced by the signal conditioning circuitry903. And FIGS. 10D through 101 depict the processing performed in one ofthe respective paths of the multi-path scan data signal processor 901.Similar processing operations with different operations characteristicsDare performed in other paths of the multi-path scan data signalprocessor 901.

[0225] More specifically, each signal processing path has differentoperational characteristics (such as different cutoff frequencies in thefiltering stages of the first and second derivative signal generationcircuits of the respective paths, different gain characteristics inamplifier stages of the first and second derivative signal generationcircuits of the respective paths, and/or different positive and negativesignal thresholds in the first derivative threshold circuitry of therespective paths). The varying operational characteristics of the pathsare optimized to provide different signal processing functions.

[0226] For example, the cut-off frequencies in the filtering stages ofthe first and second derivative signal generation circuits of therespective paths can vary such that different paths minimize the papernoise originating from different focal zones of the system.Alternatively, such cut-off frequencies can vary such that one or morepaths maximize the scan resolution of the system (i.e., a path withhigher cutoff frequencies may be able to detect high resolution bar codesymbols) while other paths minimize paper noise (i.e., a path with lowercutoff frequencies will reject paper noise from a larger frequency bandabove the selected cutoff frequencies).

[0227] In another example, the gain characteristics in the amplifierstages of the first and second derivative signal generation circuits ofthe paths and/or the positive and negative signal thresholds in thefirst derivative threshold circuitry of the paths can vary such that oneor more paths maximize the scan resolution of the system (i.e., a pathwith higher gain and/or smaller positive and negative signal thresholdsmay be able to detect low bar code symbols) while other paths minimizepaper noise (i.e., a path with lower gain and/or larger positive andnegative signal thresholds will reject paper noise that falls below suchthresholds).

[0228] The different signal processing functions of each path of themulti-path scan data processor as described above are preferablyperformed in parallel. Alternatively, the processing along each path maybe performed sequentially. In this case, a programmable microcomputermay be programmed to dynamically activate the processing of a given pathbased upon the operation of the scanner (for example, based upon thefocal distance of the scanning plane from which the scan data signal isderived, which is described in detail in U.S. application Ser. No.(108-045USA000), or based upon results of previous scan processing ofthe system).

[0229] By virtue of this improved architecture, the multi-path scan datasignal processor is able to identify signal level transitions(corresponding to transitions between a space and a bar in a bar codesymbol) in the scan data signal over a diverse range of operatingconditions (e.g., operating conditions where paper noise is present inaddition to operating conditions requiring high resolution scanning,such as the reading of low contrast or high resolution bar codesymbols), which enables more reliable bar code reading over such diverseoperating conditions.

[0230] Signal Conditioning Circuitry

[0231]FIG. 11 illustrates an exemplary embodiment of the signalconditioning circuitry 903 of FIG. 9, which operates to amplify andsmooth out or otherwise filter the scan data signal produced by thephotodetector 902 to remove unwanted noise components therein. Thecircuitry 903 comprises, a number of subcomponents arranged in a serialmanner, namely: a high gain amplifier stage 1103, a multistage amplifierstage 1105, a differential amplifier stage 1107 and a low pass filter(LPF) stage 1109. The amplifier stages 1103, 1105 and 1109 amplify thevoltage of the analog scan data signal produced by the photodetector 902with gains of 90, 3.0 and 7.1, respectively, to provide a total gain ofabout 1900. The low pass filter 1109 stage operates to filter outunwanted noise in the amplified signal produced by the amplifier stages1103, 1105 and 1109. The 3 dB cutoff frequency of the low pass filtershown (which is a maximally flat Butterworth type filter) isapproximately 780 kHz, which is designed to filter out unwanted highfrequency noise (e.g., noise which lies above the expected maximumsignal frequency of 540 kHz).

[0232] The First Derivative Signal Generation Circuitry

[0233]FIG. 12 illustrates an exemplary implementation of the firstderivative signal generation circuitry 904, which is suitable for use inthe two different paths of the scan data signal processor of FIG. 9. Asshown in FIG. 12, the first derivative signal generation circuitry 904includes a number of subcomponents arranged in a serial manner thatprocess the analog scan data signal produced by the signal conditioningcircuitry 903, namely: a differentiator stage 1201, a low-pass filter(LPF) stage 1203, and an amplifier stage 1205.

[0234] The differentiator stage 1201 generates an signal whose voltagelevel is proportional to the first derivative of the analog scan datasignal for those frequencies less than the cutoff frequency of thedifferentiator stage 1201, which is set by the values of R43 and C32,respectively, and can be approximated by the expression:${f_{c} = \frac{1}{2*\pi*{R43}*{C32}}},$

[0235] which is approximately 3.226 MHz for the circuit elements shown.

[0236] The low pass filter stage 1203 operates to filter out unwantednoise in the output signal produced by the differentiator stage 1201.The 3 dB cutoff frequency of the low pass filter shown (which is amaximally flat Butterworth type filter) is set by the values of L5 andC36, respectively, and can be approximated by the expression:${f_{c} = \frac{1}{2*\pi*\sqrt{{L5}*{C36}}}},$

[0237] which is approximately 650 kHz for the circuit elements shown.

[0238] The amplifier stage 1205 operates to amplify the voltage levelsof the output signal produced by the LPF stage for frequencies in apredetermined frequency band. More specifically, for frequencies betweenf₁ and f₂, the amplifier produces a gain that is iapproximatelyproportional to R60/R54 (which is approximately 6.5 for the circuitelements shown) where: ${f_{1} = \frac{1}{2*\pi*{R54}*{C39}}},$

[0239] which is approximately 3 kHz for the circuit elements shown.${f_{2} = \frac{1}{2*\pi*{R60}*{C43}}},$

[0240] which is approximately 2 MHz for the circuit elements shown.

[0241] Outside the predetermined frequency band between f₁ and f₂, theamplifier stage 1205 attenuates such frequency components.

[0242] It should be noted that although the first derivative signalgeneration circuitry of the two paths (labeled 904-A and 904-B in FIG.9) share a common function—to generate a signal approximating the firstderivative of the analog scan data signal—they may have differentoperational characteristics that are optimized for bar code scanning andreading in diverse operating conditions.

[0243] For example, the cut-off frequencies in the differentiator stage1201, the LPF stage 1203 and the amplifier stage 1205 of the firstderivative signal generation circuits of the respective paths (labeled904-A and 904-B) can vary (by selecting different values for theappropriate circuit elements as set forth above) such that differentpaths minimize the paper noise originating from different focal zones ofthe system. Techniques for selecting the appropriate cutoff frequenciesthat correspond to the different focal zones of the laser scanningsystem are described in detail in U.S. patent application Ser. No.(108-045USA000), commonly assigned to the assignee of the presentapplication, incorporated by reference above in its entirety.Alternatively, such cut-off frequencies can vary such that one or morepaths maximize the scan resolution of the system (i.e., a path withhigher cutoff frequencies may be able to detect high resolution bar codesymbols) while other paths minimize paper noise (i.e., a path with lowercutoff frequencies will reject paper noise from a larger frequency bandabove the selected cutoff frequencies).

[0244] In another example, the gain characteristics in the amplifierstage 1205 of the first derivative signal generation circuits of therespective paths (labeled 904-A and 904B) can vary such that one pathmaximizes the scan resolution of the system (i.e., a path with highergain may be able to detect low bar code symbols) while the other pathminimize paper noise (i.e., a path with lower gain will reject papernoise that might trigger scan errors when amplified by the high gainpath).

[0245] The Second Derivative Signal Generation Circuitry

[0246]FIG. 13 illustrate an exemplary implementation of the secondderivative signal generation circuitry 906, which is suitable for use inthe two different paths of the scan data signal processor of FIG. 9. Asshown in FIG. 13, the second derivative signal generation circuitry 906includes a differentiator stage 1301 that generates a signal whosevoltage level is proportional to the derivative of the first derivativesignal produced by the first derivative generation circuitry 904 (thusproportional to the second derivative of the analog scan data signalproduced by the signal conditioning circuitry 903) for frequencies in apredetermined frequency band. More specifically, the differentiatorstage 1301 operates substantially as a differentiator (producing asignal whose voltage level is proportional to the derivative of thefirst derivative signal produced by the first derivative generationcircuitry 904) for frequencies less than f₁ where:${f_{1} = \frac{1}{2*\pi*{R62}*{C48}}},$

[0247] which is approximately 884 kHz for the circuit elements shown.

[0248] Moreover, the feedback elements of the differentiator stage 1301operate substantially as a low pass filter with a 3 dB cutoff frequencywhich is set by the values of R65 and C49, respectively, and can beapproximated by the expression: ${f_{c} = \frac{1}{2*\pi*{R65}*{C49}}},$

[0249] which is approximately 2.15 Mhz for the circuit elements shown.

[0250] For frequencies above this predetermined 3 dB cutoff frequencyf_(c), the differentiator stage 1301 attenuates such frequencycomponents.

[0251] It should be noted that although the second derivative signalgeneration circuitry of the two paths (labeled 906-A and 906-B in FIG.9) share a common function—to generate a signal approximating the secondderivative of the analog scan data signal—they may have differentoperational characteristics that are optimized for bar code scanning andreading in diverse operating conditions.

[0252] For example, the cut-off frequencies in the differentiator stage1301 of the second derivative signal generation circuits of therespective paths (labeled 906-A and 906-B) can vary (by selectingdifferent values for the appropriate circuit elements as set forthabove) such that different paths minimize the paper noise originatingfrom different focal zones of the system.

[0253] Techniques for selecting the appropriate cutoff frequencies thatcorrespond to the different focal zones of the laser scanning system aredescribed in detail in U.S. patent application Ser. No. (108-045USA000),commonly assigned to the assignee of the present application,incorporated by reference above in its entirety. Alternatively, suchcut-off frequencies can vary such that one or more paths maximize thescan resolution of the system (i.e., a path with higher cutofffrequencies may be able to detect high resolution bar code symbols)while other paths minimize paper noise (i.e., a path with lower cutofffrequencies will reject paper noise from a larger frequency band abovethe selected cutoff frequencies).

[0254] Zero Crossing Detector

[0255]FIG. 15 illustrates an exemplary implementation of a zero crossingdetector 907, which is suitable for use in the two different paths ofthe scan data signal processor of FIG. 9. As shown in FIG. 15, thezero-crossing detector 907 includes a comparator circuit that comparesthe second derivative signal produced from the second derivativegeneration circuit in its respective path with a zero voltage reference(i.e. the AC ground level) provided by the zero reference signalgenerator, in order to detect the occurrence of each zero-crossing inthe second derivative signal,—and provide output signals (ZC_1 and ZC_2signals) identifying zero crossings in the second derivative signal.

[0256] First Derivative Signal Threshold Level Generation Circuit

[0257]FIG. 14 illustrate exemplary implementation of the firstderivative signal threshold circuitry 905, which is suitable for use inthe two different paths of the scan data signal processor of FIG. 9. Asshown in FIG. 14, the first derivative signal threshold circuitry 905includes an amplifier stage 1401 that amplifies the voltage levels ofthe first derivative signal produced by the first derivative signalgeneration circuitry 904, positive and negative peak detectors 1403 and1405, and a comparator stage 1407 that generates output signals (e.g.,the Upper_Threshold Signal and Lower_Threshold Signal) that indicate thetime period when the positive and negative peaks of the amplified firstderivative signal produced by the amplifier stage exceed predeterminedthresholds (i.e., a positive peak level PPL and a negative peak levelNPL). Preferably, the positive peak level PPL and negative peak levelNPL are dynamic thresholds (e.g., these levels change as the amplifiedanalog signal changes over time) based upon a DC bias level and apercentage (portion) of the amplified first derivative signal producedby the amplifier stage 1401. In the illustrative embodiment shown inFIG. 14, capacitors C16 and C18 are configured as peak detectors (with adecay time constant proportional to the values of R14/C16 and R19/C18,respectively); and the positive peak level PPL is set by the resistancevalues of the resistor network R16, R17, R18 and R_(U) _(—) _(BIAS),while the negative peak level NPL is set by the values of the resistornetwork R21, R22, R23 and R_(L) _(—) _(BIAS).

[0258] It should be noted that although the first derivative signalthreshold circuitry of the two paths (labeled 905-A and 905-B in FIG. 9)share a common function—to generate output signals that indicate thetime period when the positive and negative peaks of the amplified firstderivative signal exceed predetermined thresholds—they may havedifferent operational characteristics that are optimized for bar codescanning and reading in diverse operating conditions.

[0259] For example, the positive and negative peak levels in thepositive and negative peak detectors 1403 and 1405, respectively, (whichare set by the resistance values of the resistor networks therein) canvary such that one path maximizes the scan resolution of the system(i.e., a path with lower positive peak and negative peak level may beable to detect low bar code symbols) while the other path minimize papernoise (i.e., a path with a higher positive peak and negative peak levelwill reject paper noise that that falls below such thresholds.

[0260] For example, the positive and negative peak detectors 1403 and1405 in the first derivative signal threshold circuitry 905-A of thefirst path A may utilize a 91 kilo-ohm resistor for R_(U) _(—) _(BIAS)and R_(L) _(—) _(BIAS) of FIG. 14. Such resistor values produce adynamic PPL threshold which approximates 2.079 mV DC bias level plus 24%of the amplified first derivative signal, and produce a dynamic NPLthreshold which approximates a 1.921 mV DC bias level less 24% of theamplified first derivative signal. In another example, the positive andnegative peak detectors 1403 and 1405 in the first derivative signalthreshold circuitry 905-B of the second path B may utilize a 20 kilo-ohmresistor for R_(U) _(—) _(BIAS) and R_(L) _(—) _(BIAS) of FIG. 14. Suchresistor values produce a dynamic PPL threshold which approximates a2.316 mV DC bias level plus 21% of the amplified first derivativesignal, and produce a dynamic NPL threshold which approximates 1.684 mVDC bias level less 21% of the amplified first derivative signal. Notethat path A has “lower” positive peak and negative peak levels—it may beable to detect high resolution bar code symbols than path B. While pathB has “higher” positive peak and negative peak levels—it will rejectpaper noise that might trigger scan errors in the path A).

[0261] Data Gating Circuitry and 1-Bit A/D Conversion Circuitry

[0262]FIG. 16 illustrates an exemplary implementation of the data gatingcircuitry 908 and 1-Bit IA/D conversion circuitry 909, which is suitablefor use in the two different paths of the scan data signal processor ofFIG. 9. In each respective path, the data gating circuit 908 functionsto gate to the binary-type A/D signal conversion circuitry 909, onlydetected second derivative zero-crossings (identified by the outputssignals ZC_1 and ZC_2 of the zero crossing detector 907 in therespective path) which occur substantially concurrent to a positive ornegative peaks detected in the “first derivative signal” (as identifiedby the output signals—Upper_Threshold and Lower_Threshold—of the firstderivative threshold circuitry 905). As shown in FIG. 16, the data gatecircuit 908 and the 1 bit D/A conversion circuitry 909 in each path isrealized by four NAND gates (labeled 601 through 1604) configured as aset/reset latch circuit. The operation of the data gating circuitry and1 bit D/A conversion circuitry of FIG. 16 is illustrated in the signalplot of FIG. 101.

[0263] Having described illustrative embodiments of the presentinvention, it is understood that there a number of alternative ways topractice the present invention. Several different modes for carrying outthe present invention will be described below.

[0264] For example, rather than using “analog-type” circuit technologyfor realizing the signal processing subcomponents of the multi-path scandata signal processor (e.g., the differentiators, low-pass filter,amplifiers, peak detectors, data gate, etc.), it is understood that thescan data signal processing method and apparatus of the presentinvention can be implemented using digital signal processing techniquescarried out either within a programmed microcomputer or using one ormore custom or commercially available digital signal processing (DSP)chips known in the digital signal processing art.

[0265] As illustrated in FIG. 17A, when carrying out a digitalimplementation of the scan data signal processor of the presentinvention, the analog scan data signal D₁ is provided to signalconditioning circuitry 1703 (which amplifies and filters the signal toremove unwanted noise components as described above), whose output isprovided to analog-to-digital conversion circuitry 1705. Theanalog-to-digital conversion circuitry 1705 samples the conditionedanalog scan data signals at a sampling frequency at least two times thehighest frequency component expected in the analog scan data signal, inaccordance with the well known Nyquist criteria, and quantizes eachtime-sampled scan data signal value into a discrete signal level using asuitable length number representation (e.g. 8 bits) to produce adiscrete scan data signal. A suitable quantization level can be selectedin view of expected noise levels in the signal. Thereafter, the discretescan data signal is processed by the programmed processor (e.g., adigital signal processor 1707 and associated memory 1709 as shown) togenerate a sequence of digital words (i.e. a sequence of digital countvalues) D₃, each representing the time length associated with the signallevel transitions in the corresponding digital scan data signal asdescribed above. Preferably, these digital count values are in asuitable digital format for use in carrying out various symbol decodingoperations which, like the scanning pattern and volume of the presentinvention, will be determined primarily by the particular scanningapplication at hand.

[0266]FIGS. 17B through 17D illustrate exemplary digital implementationsof the multi-path scan data processing according to the presentinvention. The digital signal processing operations therein arepreferably carried out on the discrete scan data signal levels generatedby the A/D converter 1705 and stored in the memory 1709 of FIG. 17A.

[0267]FIG. 17B illustrates exemplary digital signal processingoperations that identify a data frame (e.g., a portion of the discretescan data signal levels stored in memory 1709) that potentiallyrepresents a bar code symbol (block 1723) and stores the data frame in aworking buffer (block 1725). Signal processing techniques that identifya data frame (within the discrete signal levels stored in the memory1709) that potentially represents a bar code symbol (block 1723) arewell know in the art.

[0268]FIG. 17C illustrates exemplary digital signal processingoperations that carry out multi-path scan data signal processingaccording to the present invention. More specifically, in block 1727, adata frame is read from the working buffer. Preferably, the data frameread from the working buffer in block 1727 was stored therein in block1725 of FIG. 17B. Alternatively, the data frame may be a block of thediscrete scan data signals levels generated by the A/D converter E1705and stored in memory 1709 of FIG. 17A (or discrete scan data signalsderived therefrom). The data values of the data frame are then processedby a sequence of signal processing blocks (blocks 1729, 1731-1745 and1751-1765).

[0269] In block 1729, such data values are optionally interpolated (orsub-sampled). Interpolation increases the effective sampling rate of thesystem by adding data values that are derived from existing data values.Interpolation is a technique well known in the digital signal processingarts, and is discussed in great detail in Russ, “Image ProcessingHandbook,” Third Edition, IEEE Press, 1999, pg. 219-220, hereinincorporated by reference in its entirety. Sub-sampling (or decimation)decreases the effective sampling rate of the system. Sub-sampling istypically accomplished by averaging data values. Sub-sampling is atechnique well known in the digital signal processing arts, and isdiscussed in great detail in Russ, “Image Processing Handbook,” ThirdEdition, IEEE Press, 1999, pg. 166-174, herein incorporated by referencein its entirety. The resulting block of data values are provided to atleast two processing paths (for example, two paths A and B as shown).The different digital signal processing functions of each path arepreferably performed in parallel (for example, by separated threads in amulti-threaded processing system or by separate processors in amulti-processor system). Alternatively, the processing along each pathmay be performed sequentially.

[0270] In each respective processing path, the block of data values aresubject to a digital low pass filter (blocks 1731 and blocks 1753) thatfilter out unwanted noise. Such digital low-pass filters preferablyimplement one of a Butterworth-type, Chebsychev-type, MFTD-type, orelliptical-type low pass filtering transfer function, which are wellknown in the filtering art. Details of the design of such digitalfilters is set forth in the book entitled “Electrical Filter DesignHandbook,” Third Edition, by A. Williams et al. McGraw-Hill, 1996,incorporated by reference above in its entirety. The output of thedigital low pass filter (blocks 1731, 1751) is supplied to a firstderivative processing function (blocks 1733, and 1753) whichdifferentiate the filtered digital scan data signals supplied thereto.The output of the first derivative processing function (blocks 1733,1753) is normalized (blocks 1734, 1754) and supplied to a firstderivative thresholding function (blocks 1739 and 1759) and a secondderivative processing function (blocks 1735 and 1755).

[0271] The second derivative processing function (blocks 1735, 1755)differentiates the data supplied thereto to generate data representingthe second derivative of the data values read from the working buffer.Such data is supplied to a zero crossing detector function (blocks 1737,1757), which produces output data (“zero crossing data”) identifyingzero crossings in the second derivative data generated by the secondderivative function (blocks 1735, 1755).

[0272] The first derivative thresholding function (blocks 1739, 1759)operates as a positive and negative peak detector to provide output datathat identifies time periods when the positive and negative peaks of thedata supplied thereto exceed predetermined thresholds (i.e., a positivepeak level PPL and a negative peak level NPL). Preferably, the positivepeak level PPL and negative peak level NPL are dynamic thresholds (e.g.,these levels change as the digital scan data values read from theworking buffer change over time) based upon a predetermined digitalvalue and a percentage (portion) of the corresponding normalized firstderivative signal supplied thereto.

[0273] The data output of the zero crossing detector function (blocks1737, 1757) and the first derivative thresholding function (blocks 1739,1759) are supplied to a data gate function (blocks 1741, 1761), whichfunctions to output only zero crossing data which corresponds todetected zero-crossings which occur substantially concurrent with thepositive or negative peaks detected in the normalized first derivativedata (as identified by the output data of the first derivative thresholdfunction). Thereafter, the data output by the data gate function (whichrepresents a discrete binary-level scan data signal) is supplied to abar length function (blocks 1743, 1763), which produce a digital “time”count value for each of the first and second signal levels in thediscrete binary scan data signal. Such digital count values form asequence of digital word D₃, leach representing the time lengthassociated with the signal level transitions in the correspondingdigital scan data signal as described above. These digital words arestored in an output buffer (blocks 1745, 1765), for supply to aprogrammed decoder for decoding the scan data signal and producingsymbol character data string representative of the correspondinglaser-scanned bar code symbol. Alternatively, the generated discretebinary-level scan data signal can be converted back into acontinuous-type binary-level scan data signal so that it may be“digitized” using a digital signal processor of the type taught in U.S.Pat. No. 5,828,049, incorporated herein by reference.

[0274] Each digital signal processing path has different operationalcharacteristics (such as different cutoff frequencies in the low passfilters (blocks 1731 and 1751) and/or different positive and negativesignal thresholds in the first derivative threshold function (blocks1739, 1759) of the respective paths). The varying operationalcharacteristics of the paths are optimized to provide different digitalsignal processing functions.

[0275] For example, the cut-off frequencies in the low pass filters(blocks 1731 and 1751) of the respective paths can vary such thatdifferent paths minimize the paper noise originating from differentfocal zones of the system. Alternatively, such cut-off frequencies cansuch that vary such that one or more paths maximize the scan resolutionof the system (i.e., a path with higher cutoff frequencies may be ableto detect high resolution bar code symbols) while other paths minimizepaper noise (i.e., a path with lower cutoff frequencies will rejectpaper noise from a larger frequency band above the selected cutofffrequencies).

[0276] In another example, the positive and negative signal thresholdsin the first derivative threshold functions (blocks 1739, 1759) of therespective paths can vary such that one or more paths maximize the scanresolution of the system (i.e., a path with “smaller” positive andnegative signal thresholds may be able to detect low contrast bar codesymbols) while other paths minimize paper noise (i.e., a path with a“larger” positive and negative signal thresholds will reject paper noisethat falls below such thresholds.

[0277] The different digital signal processing functions of each path asdescribed above are preferably performed in parallel (for example, byseparated threads in a multi-threaded processing system or by separateprocessors in a multi-processor system). Alternatively, the processingalong each path may be performed sequentially. In this case, theprogrammable microcomputer (e.g., digital signal processing system) maybe programmed to dynamically activate the processing of a given pathbased upon the operation of the scanner (for example, based upon thefocal distance of the scanning plane from which the scan data signal isderived, which is described in detail in U.S. application Ser. No.(108-045USA000), or based upon results of previous scan processing ofthe system.

[0278]FIG. 17D illustrates alternative digital signal processingoperations that carry out multi-path scan data signal processingaccording to the present invention. More specifically, in block 1771, adata frame is read from the working buffer. Preferably, the data frameread from the working buffer in block 1727 was stored therein in block1725 of FIG. 17B. Alternatively, the data frame may be a block of thediscrete scan data signals levels generated by the A/D converter 1705and stored in memory 1709 of FIG. 17A (or discrete scan data signalsderived therefrom). In block 1773, such data values are optionallyinterpolated (or sub-sampled). Interpolation increases the effectivesampling rate of the system by adding data values that are derived fromexisting data values.

[0279] In block 1775, the resulting block of data values are subject toa digital low pass filter that filters out unwanted noise. Such digitallow-pass filter preferably implements one of a Butterworth-type,Chebsychev-type, MFTD-type, or elliptical-type low pass filteringtransfer function, which are well known in the filtering art. Details ofthe design of such digital filters is set forth in the book entitled“Electrical Filter Design Handbook,” Third Edition, by A. Williams etal. McGraw-Hill, 1996, incorporated by reference above in its entirety.The output of the digital low pass filter (block 1775) is supplied to afirst derivative processing function (block 1777) which differentiatesthe filtered digital scan data signals supplied thereto. The output ofthe first derivative processing function (block 1777) is normalized(block 1779) and supplied to a second derivative processing function(block 1781).

[0280] The second derivative processing function (block 1781)differentiates the data supplied thereto to generate data representingthe second derivative of the data values read from the working buffer.Such data is supplied to a zero crossing detector function (block 1783),which produces output data (“zero crossing data”) identifying zerocrossings in the second derivative data generated by the secondderivative function.

[0281] The normalized output of the first derivative processing function(block 1779) is also supplied to at least one processing sub-path (forexample, sub-path A as shown). In the illustrative embodiment shown inFIG. 17D, the execution of the signal processing of the second sub-pathB is contingent upon a status condition of the working buffer (e.g.,whether it has (or has not) received another full data frame.Alternatively, the different digital signal processing functions of eachsub-path may be performed in parallel (for example, by separated threadsin a multi-threaded processing system or by separate processors in amulti-processor system).

[0282] Each processing sub-path includes a first derivative thresholdingfunction (blocks 1785, 1795), which operates as a positive and negativepeak detector to provide output data that identifies time periods whenthe positive and negative peaks of the data supplied thereto exceedpredetermined thresholds (i.e., a positive peak level PPL and a negativepeak level NPL). Preferably, the positive peak level PPL and negativepeak level NPL are dynamic thresholds (e.g., these levels change as thedigital scan data values read from the working buffer change over time)based upon a predetermined digital value and a percentage (portion) ofthe corresponding normalized first derivative signal supplied thereto.

[0283] The data output of the zero crossing detector function (block1783) and the first derivative thresholding function of the respectivepath (block 1785, 1795) are supplied to a data gate function (blocks1787, 1797), which functions to output only zero crossing data whichcorresponds to detected zero-crossings which occur substantiallyconcurrent with the positive or negative peaks detected in thenormalized first derivative data (as identified by the output data ofthe first derivative threshold function). Thereafter, the data output bythe data gate function (which represents a discrete binary-level scandata signal) is supplied to a bar length function (blocks 1789, 1798),which produce a digital “time” count value for each of the first andsecond signal levels in the discrete binary scan data signal. Suchdigital count values form a sequence of digital word D₃, eachrepresenting the time length associated with the signal leveltransitions in the corresponding digital scan data signal as describedabove. These digital words are stored in an output buffer (blocks 1791,1799), for supply to a programmed decoder for decoding the scan datasignal and producing symbol character data string representative of thecorresponding laser-scanned bar code symbol. Alternatively, thegenerated discrete binary-level scan data signal can be converted backinto a continuous-type binary-level scan data signal so that it may be“digitized” using a digital signal processor of the type taught in U.S.Pat. No. 5,828,049, incorporated herein by reference.

[0284] Each digital signal processing sub-path of FIG. 17D has differentoperational characteristics (such as different positive and negativesignal thresholds in the first derivative threshold function (blocks1785, 1795) of the respective sub-paths). The varying operationalcharacteristics of the sub-paths are optimized to provide differentdigital signal processing functions.

[0285] For example, the positive and negative signal thresholds in thefirst derivative threshold functions (blocks 1785, 1795) of therespective sub-paths can vary such that one or more sub-paths maximizethe scan resolution of the system (i.e., a sub-path with “smaller”positive and negative signal thresholds may be able to detect low barcode symbols) while other sub-paths minimize paper noise (i.e., asub-path with a “larger” positive and negative signal thresholds willreject paper noise that falls below such thresholds.

[0286] Note that the illustrative embodiments set forth above provide amulti-path scan data signal processor with two signal processing paths(or sub-paths) with different operational characteristics. It iscontemplated that the multi-path scan data signal processor of thepresent invention includes more than two signal processing paths (orsub-paths) with different operational characteristics as describedabove.

[0287] Advantageously, the scan data signal processor of the presentinvention has an improved signal-to-noise ratio (SNR) and dynamic range,which effectively increases the length of each focal zone in the system.This allows the system designer to provide more overlap between adjacentfocal zones or produce a laser scanning system with a larger overalldepth of field. In addition, it produces a laser scanning system capableof scanning/resolving bar code symbols having narrower element widthsand/or printed on substrates whose normal vector is disposed at largeangles from the projection axis of scanning system.

[0288] Several modifications to the illustrative embodiments have beendescribed above. It is understood, however, that various othermodifications to the illustrative embodiment of the present inventionwill readily occur to persons with ordinary skill in the art. All suchmodifications and variations are deemed to be within the scope andspirit of the present invention as defined by the accompanying Claims toInvention.

What is claimed is:
 1. A bioptical laser scanning device comprising: abottom-scanning window substantially orthogonal to a side-scanningwindow; at least one scanning element that cooperates with a pluralityof laser beam folding mirrors to produce a plurality a horizontalscanning planes that project from exterior portions of the side-scanningwindow at a characteristic propagation direction whose non-verticalcomponent is greater than thirty-five degrees from normal of theside-scanning window.
 2. The bioptical laser scanning device of claim 1,wherein said plurality of horizontal scanning planes include at leastone group of horizontal scanning planes that project from the exteriorleft portion of the side-scanning window and include at least one groupof horizontal scanning planes that project from the exterior rightportion of the side-scanning window.
 3. A bioptical laser scanningdevice comprising: a bottom-scanning window substantially orthogonal toa side-scanning window; at least one scanning element that cooperateswith a plurality of laser beam folding mirrors to produce a plurality avertical scanning planes that project from portions of thebottom-scanning window proximate to the back of the bottom-scanningwindow and the bottom of the side-scanning window.
 4. The biopticallaser scanning device of claim 1, wherein the vertical scanning planesproject from back-left and back-right corners of the bottom-scanningwindow that are proximate to the bottom of the side-scanning window. 5.A bioptical laser scanning device comprising: a bottom-scanning windowsubstantially orthogonal to a side-scanning window, wherein thebottom-scanning window has four corners; at least one scanning elementthat cooperates with a plurality of laser beam folding mirrors toproduce a plurality a vertical scanning planes that project from eachone of the four corners of the bottom-scanning window.
 6. A biopticallaser scanning device comprising: a bottom-scanning window substantiallyorthogonal to a side-scanning window; at least one scanning element thatcooperates with a plurality of laser beam folding mirrors to produce aplurality of groups of vertical scanning planes that project from thebottom-scanning window, wherein said plurality of groups include firstand second groups of vertical scanning planes that project from opposingportions of the bottom-scanning window, and said plurality of groupsinclude third and fourth groups of vertical scanning planes, differentfrom said first and second groups, that project from opposing portionsof the bottom-scanning window.
 7. The bioptical laser scanning device ofclaim 6, wherein the first and second groups project from the back-leftand front-right corners, respectively, of the bottom-scanning window,and wherein the third and fourth groups project from the back-right andfront-left corners, respectively, of the bottom-scanning window.
 8. Thebioptical laser scanning device of claim 6, wherein the first and secondgroups of vertical scanning planes are substantially co-planar, andwherein the third and fourth groups of vertical scanning plane aresubstantially co-planar.
 9. The bioptical laser scanning device of claim6, wherein the first and second groups of vertical scanning planes aresubstantially orthogonal to the third and fourth groups of scanningplanes.
 10. A bioptical laser scanning device comprising: abottom-scanning window substantially orthogonal to a side-scanningwindow; at least one scanning element that cooperates with a pluralityof groups of laser beam folding mirrors to produce a plurality avertical scanning planes projecting from the bottom-scanning window thatare capable of reading bar-code symbols on the bottom surface and allfour sides of a four-sided article.
 11. A laser scanning devicecomprising at least one window; and at least one laser beam productionmodule that cooperates with a rotating polygonal mirror and a pluralityof laser beam folding mirrors to produce a plurality of scanning planesthat project through the window, wherein the incidence angle of thelaser beam produced by the laser beam production module is offset withrespect to the axis of rotation of the rotating polygonal mirror. 12.The laser scanning device of claim 11, further comprising a plurality oflaser beam production modules that cooperate with the rotating polygonalmirror and the plurality of laser beam folding mirrors to produce aplurality of scanning planes that project through the window, whereinthe incidence angle of the laser beam produced by each laser beamproduction module is offset with respect to the axis of rotation of therotating polygonal mirror to produce overlapping scanning ray patternsthat are incident on at least one common mirror to provide a densescanning pattern projecting therefrom.
 13. The laser scanning device ofclaim 12, wherein the window comprises one of a bottom-scanning windowand a side-scanning window of a bioptical laser scanner.
 14. The laserscanning device of claim 13, wherein the common mirror redirects thedense scanning pattern incident thereon to form a plurality ofhorizontal scan planes that project from the front side of thebottom-scanning window diagonally upward toward the side-scanningwindow.
 15. A bioptical laser scanning device comprising: abottom-scanning window substantially orthogonal to a side-scanningwindow; at least one scanning element that cooperates with a pluralityof laser beam folding mirrors to produce at least eight differentvertical scanning planes that project from the side-scanning window. 16.The bioptical laser scanning device of claim 15, wherein theside-scanning window with a square area less than 37,500 sq. mm.
 17. Abioptical laser scanning device comprising: a bottom-scanning windowsubstantially orthogonal to a side-scanning window; at least onescanning element that cooperates with a plurality of laser beam foldingmirrors to produce at least 13 different horizontal scanning planes thatproject from the side-scanning window.
 18. The bioptical laser scanningdevice of claim 17, wherein the at least one scanning element cooperateswith said plurality of laser beam folding mirrors to produce at least 20different horizontal scanning planes that project from the side-scanningwindow.
 19. The bioptical laser scanning device of claim 17, wherein theside-scanning window with a square area less than 37,500 sq. mm.
 20. Abioptical laser scanning device comprising: a bottom-scanning windowsubstantially orthogonal to a side-scanning window; at least onescanning element that cooperates with a plurality of laser beam foldingmirrors to produce at least 21 different scanning planes that projectfrom the side-scanning window.
 21. The bioptical laser scanning deviceof claim 20, wherein the at least one scanning element cooperates withsaid plurality of laser beam folding mirrors to produce at least 28different scanning planes that project from the side-scanning window.22. The bioptical laser scanning device of claim 20, wherein theside-scanning window with a square area less than 37,500 sq. mm.
 23. Abioptical laser scanning device comprising: a bottom-scanning windowsubstantially orthogonal to a side-scanning window; at least onescanning element that cooperates with a plurality of laser beam foldingmirrors to produce at least seven different vertical scanning planesthat project from the bottom-scanning window.
 24. The bioptical laserscanning device of claim 23, wherein the bottom-scanning window with asquare area less than 15,000 sq. mm
 25. A bioptical laser scanningdevice comprising: a bottom-scanning window substantially orthogonal toa side-scanning window; at least one scanning element that cooperateswith a plurality of laser beam folding mirrors to produce at least 21different horizontal scanning planes that project from thebottom-scanning window.
 26. The bioptical laser scanning device of claim25, wherein the at least one scanning element cooperates with saidplurality of laser beam folding mirrors to produce at least 24 differenthorizontal scanning planes that project from the bottom-scanning window.27. The bioptical laser scanning device of claim 25, wherein thebottom-scanning window with a square area less than 15,000 sq. mm.
 28. Abioptical laser scanning device comprising: a bottom-scanning windowsubstantially orthogonal to a side-scanning window; at least onescanning element that cooperates with a plurality of laser beam foldingmirrors to produce at least 25 different scanning planes that projectfrom the bottom scanning window.
 29. The bioptical laser scanning deviceof claim 28, wherein the at least one scanning element cooperates withsaid plurality of laser beam folding mirrors to produce at least 40different scanning planes that project from the bottom-scanning window.30. The bioptical laser scanning device of claim 28, wherein thebottom-scanning window with a square area less than 15,000 sq. mm.